Today is the last day of my current thrift contest, so get on over there and enter before midnight if you want a chance at a custom-thrifted prize!
Now, as promised, my answer to the question "Why do you thrift?"
I thrift for all the reasons people have mentioned so far--frugality, environmentalism, preservation of history, the thrill of the chase. My biggest reason, however, is different than those mentioned so far:
I thrift in order to get time alone. I am a person who needs a lot of alone time who lives with a homebody. I am very, very rarely home alone. So I need to get out of the house for my alone time, and my number one favorite thing to get out and do, by myself, is shop.
I know it's out of fashion to admit you like to shop, at least for my social/political group. I'm supposed to claim to hate shopping and be sickened by the crowds/consumerism/exchange of money. But in reality, I don't, and I'm not. I absolutely love to shop. Walking around, looking at things, doing something in a public space that is totally acceptable to partake in alone--I just adore it.
Thrifting allows me to fulfill my desire to shop in a way that is both more economically viable and more environmentally friendly than any other type of shopping I could be doing. This is not only due to the lower cost and reduced environmental impact of thrifted goods, but also due to the time in to time out equation. When you are shopping secondhand, especially somewhere like the bins, it takes a lot more time and effort to find things you want to buy than it might in a more shopper-friendly environment. The treasure hunt angle. Because shopping at a thrift store is more challenging than a first-run store might be, I can spend the same amount of time shopping, but come home with less stuff. Because for me the actual experience of shopping, rather than what I find, is the point, that's a win-win situation.
As far as I'm concerned, there is no bad reason to thrift shop. The more people accept secondhand goods as viable options, the better off we'll be, in terms of our environment, our wallets, and the support we're (not) giving to crappy labor practices. I continually strive to buy fewer non-used items. There are so many reasons to do it, and I can't think of a single good reason not to.