I imagine it's not a very common question for members of either of the two major parties. Most of the time people snap to whatever pre-conceived judgment they have about the party, and then sort Republicans and Democrats into little bins in their mind with labels like, "elitist", "culture warrior", or even "racist".
But because I am Libertarian, I have a unique opportunity to share my reasoning free of worry!
It boils down to this:
I am Libertarian because, once a year, right after the calendar rolls over, I have to do my taxes. And every year, when I look at how much of my money has been silently taken from me, I have to ask myself, is my life better off now than it was last year because of this? Was my money-- which was taken by force-- better spent on bailouts, subsidies, and all manner of crony corporatism? Of course not!
The idea of that your money is taken by force, and then handed over to corporate cronies is shocking! And yet, every day, through the Import-Export bank, the Federal Reserve, and acts of Congress, your money is doled out to big corporations like Boeing, AIG, GM, Goldman Sachs, Solyndra... and too many more to count.
The sad part about this is that it is a result of bipartisan cooperation. Indeed, the unprecedented level of crony corporatism to which we are now enslaved could only be achieved through broad support from both Republicans and Democrats. And, as the recent $9 billion dollar tax break for Boeing shows, it remains alive and well in Washington state.
I do not endorse theft in any form, but taxation is particularly egregious because it is the benefactor of so much evil.
I don't want my money spent on evil things. That is why I am Libertarian.