Although I'm not quite a luminary up there with those quoted in Sean's post, I'm going to give two answers.
The first answer, of course, is very personal, and was represented by the huge life change I went through in 2007. Specifically, I decided that a career in the research and teaching of physics and astronomy wasn't worth it. I was at the wrong place (a research University) at the wrong time (when it was getting more difficult to get NSF funding) which made it really the wrong place (the research University was inflexible enough that funding was a tenure veto criterion), and I failed to move to the right place (I interviewed at a few liberal arts colleges I would have loved to go to, but either didn't get offers, or in one case got an offer that was not feasable for me). I loved the teaching of physics and astronomy, and loved the science, but finally was able to admit to myself that it had been a few years since that love justified putting up with the crap. The main reason I hadn't admitted that to myself was that to leave was to be a failure. (Indeed, today, I still feel a bit like a failure because I couldn't make it in academia.) However, I've moved on to something else which I'm finding stimulating and rewarding. I'd thought about letting go a few times before; this was the time when I was able to bring my mind around enough to actually letting go. I'm glad I did it before my hand was forced by being fired. (Many not in academia don't realize that being denied tenure means that you must leave the institution where you are, but that is the case.)
The second answer I have is more longwinded, and is a change of mind that happened over the last six or seven years. Specifically: some form of socialized medicine is necessary for the good of the United States. If you had asked me six years ago my opinions about socialized medicine, I would have repeated a lot of the arguments you hear from others who are opposed to it: freedom to choose your doctor and other health care providers, long wait time for non-immediate-emergency procedures in some countries that currently have socialized medicine, the typical inefficency of anything that is government controlled, the quashing of innovation that comes whenever anything is moved out of the free market and into a centrally controlled bureaucracy.
And, the truth is, I still believe a lot (if not all) of those things. However, until eight or nine years ago, I was in the situation that a lot of Americans are in. I was young and healthy (or healthy enough). I had no major illnesses. I had health insurance through (first) my graduate school or (later) my job, and as such was able to get what care I needed. I didn't worry too much, because if I got a contracting job, or something else that didn't pay health insurance, it was possible to get personal health insurance. (I know a few people who do that.)
I married somebody with a chronic illness. Somebody who will not be able to get health insurance-- because it makes no business sense whatsoever to give it to her-- unless she's in a large group plan, where the power of group bargaining is able to bring her into that plan. She also isn't able to work a full time job; the fatigue and restrictions that comes with her illness make it very challenging for her to even work a quarter time. What this means is that I, as her husband, must remain in a job that's got a group health insurance plan, or we're screwed.
For most middle-class Americans without children, I suspect that housing (i.e. mortgage or rent) is the dominant single expense they have. In our house, even after health insurance, medical needs is our dominant single expense. This is part of why I wasn't able to take the job offer I had at a small liberal arts college a couple of years ago; I couldn't afford the 30% pay cut that taking that job would have represented. It is also part of why I'm very anxious and afraid about making any kind of job change. Everybody is anxious and afraid about losing their job, because of the uncertainty and fear of insecurity that comes with it. People like me-- people with uninsurable family members-- have the additional anxiety that we must be able to find a job that includes a group health insurance plan. I worry about getting fired from my current job-- not just for the reasons that everybody worries, but also because I would absolutely need to find a new job as fast as possible that my wife and I wouldn't find ourselves in the financially untenable position of having a gap in insurance that would cause "pre-existing condition" clauses to kick in on whatever policy we had with the new job. This isn't just me; many, many people are in the same situation as my family.
The current situation hurts small businesses too-- the sort of thing that free markets are supposed to be the biggest supporter of. My aunt runs an ice cream store, and has for many years. She often find it exhausting, unsurprisingly. She would love to find somebody she could hire on and keep as a long-term manager, but she's had a lot of trouble doing this. Several years ago, when talking to her about this, and feeling (as I always did) stressed and unhappy with academia, I briefly fantasized about moving to Charleston, WV, working as a manager at an ice cream shop, and having a much lower stress life. The problem is, my aunt can't hire on somebody that is really good enough to be a manager who will stay for long, because she would need to offer them health insurance... and at a store with a handful of employees, that's simply not possible. So, the best people she hires tend to be short-term... students and others who have plans to move on to bigger and better things. Right now, security in health insurance is coupled to having somebody in the family working at a large enough employer to have a group health insurance plan. This means that somebody who might (for other reasons) decide to take a job with a very small business may not practically speaking have the freedom of choice to do so.
The free market is a great solution to many things. It's far more flexible than a centrally managed solution. In many cases, if you try to dictate the solution ahead of time, you ossify your society, writing in strictures for how things work that may not be the best, and may not keep up with the change of society and technology. If you have a free market, the market can adapt to new solutions naturally, without having to convince people wedded to the old ways of doing things to change the laws and government procedures. For a lot of things, this is great. But, to many libertarians, the free market is the hammer that makes everything in the world look like a nail. While the free market may do the greatest good to the greatest number of people in many cases, when it comes to health insurance, working out a good solution for the majority of people mean that those not in the majority suffer and die. It's not a matter of providing the best entertainment options, of providing the kinds of cars people want, of providing the food choices that people are willing to pay for; it's a matter of sickness and health, and of life and death.
And, we don't even really have a free market solution right now. The system we have in this country right now mixes all of the worst aspects of free-market health care and socialized medicine. One of the biggest worries with socialized medicine is that we'd be turning over our health care to the same people who run the IRS, to the same people who run the DMV, to the same legions of politicians and faceless bureaucrats that haven't been able to balance the budget for years. People would end up cogs in the system, without the freedom to make intelligent choices about health care providers themselves. But think about it: how different is that, really, from what we have right now? If you're in a group health insurance plan, you are at the mercy of the "preferred provider" list and the formulary of that plan. If you're like me, and only occasionally have to go to the doctor, you don't notice that the system of insurance we have is saddled with soulless bureaucracies as much as any government agency... but it is. My wife, exhausted already, spends far too much time talking to health care providers trying to figure out how to get this or that thing approved, trying to figure out what changes were made to the drug formulary this year. And, all of the expected symptoms of bureaucratic bloat are there. Byzantine and complicated rules. Different answers every time you get somebody different on the phone. Uncertainty as to whether or not something you need will really be covered. Backtracking as the insurance company screws something up and tries to tell you that what you didn't isn't allowed. And, finally, for some things, paying a full out-of-pocket price because (for example) psychological health care is never covered at the level of physical health care.
In many ways, we've already saddled ourselves with a lot of the bogey men that those against socialized medicine scare us with. At the same time, we have the fundamental problem of a free market health care solution : people falling between the cracks. Those who aren't basically healthy being left behind by a system that recognizes that it is simply not economically sensible to keep them alive.
Now, mind you, we could very easily -- and, I fear, probably will -- do socialized medicine dreadfully wrong. If and when we implement some form of universal health care in this country, I think we should look to how the framers of the Constitution implemented government. We would need some form of separation of powers (with checks and balances), and we would need some explicit declarations of individuals rights. In particular, the organization or agency that pays for health insurance must be explicitly separated from the organization or agency that dictates what is covered. Without that, budget constraints will inevitably lead to the same nightmare of capricious and mercurial lists of what and who is covered that we have in our current system.
If I were president, the very first thing I'd do is outlaw pharmaceutical patents, coupled with a raise in the budget for pharmaceutical research. This would very likely spin the country into a huge recession, because so many of our mutual funds are heavily invested in profitable drug companies right now... but we'd come out a stronger and more moral place on the other side. Pharmaceutical companies argue that drug patents, and the exclusive rights that those companies receive allowing them to sell drugs for the mint they are sold for, are necessary to fund research. Of course, it turns out that those patents are funding advertising more than research. We don't need pharmaceutical companies to develop drugs; what we need are the drug researchers and those who directly support them. If the chemists and biologists who develop new drugs can make a reasonable living doing that with publicly funded research, we will still get the new drugs that we get right now... without having to sign over control over the pricing of these new drugs to a soulless entity (a large corporation) that's only interested in leveraging its assets for maximum profit. As a side benefit, we'd no longer be in the position our country has been in in recent years, having to argue that people in Africa need to die in order to "protect the intellectual property" of the USA. (AIDS drugs can be very expensive, and many impoverished nations in Africa can't afford them. The USA has in the past argued explicitly against ways to provide affordable drugs to these populations, because to do so would have been to allow production of those drugs in violation of the patents granted to the US companies that developed them. This is not the behavior of a moral nation.)
So, yes, many years ago I was opposed to socialized medicine because of (a) fear of ever growing government control, (b) the inefficiency and threat to innovation represented by large bureaucracies, and (c) freedom of choice. It took being married to somebody who really has to interact with our current health care system all the time to recognize that for many in this country, we don't entirely have any of (a), (b), or (c) with our current system... and we do have people left behind by that system.