I have gotten this question a lot, and someone just asked me to answer it yesterday so I figure I will just document my story here so I can just send a link to people instead of repeating everything. Here it goes:
I was raised vegetarian. My father came from a Hindu background and when he came to the United States he became an atheist eventually, but kept vegetarianism in his code of ethics. As I was growing up it was always understood that I shouldn't eat meat, never really forced on me or anything, but that was just how it went. For the first 10 years of my life I didn't eat meat, not really knowing why I didn't just kinda going with it.
I turned 10, and my parents divorced. My dad moved away and I lived with my mom. She was not a vegetarian and after constant pressure from my friends and my lack of reasoning for vegetarianism (if you don't know me, I don't like to do things unless I know WHY I am doing them) I cracked and had a piece of chicken at some party at my mom's friend's house. This began, the "dark ages" of my life.
My little brother had gone through the same process, except that he was 7 instead of 10. After about a year of meat eating, he decided to become a vegetarian on his own (smart kid). I went on for 5 years. During the years of overlap, when he was a vegetarian and I was not, I used to give him a ton of crap for it (I was an awesome kid).
Then I met someone who was not raised vegetarian but made the conscious decision to do so at a later point in his life. This was Freshman year of high school. I guess I had met people like this before, but this kid was kinda preachy and I actually listened to him. All of a sudden something snapped in me. I was killing animals for food unnecessarily. I knew for a fact that a vegetarian diet wasn't that hard, I had done it for 10 years already, so I began to try and rationalize why to eat meat. I went and thought that it would be healthier to eat meat, and then upon research found that the average vegetarian lives 7 years longer and has less risk of obesity, heart disease and some cancers. So that didn't work. I may have tried a few other rationalizations, but the only one that worked was "it tastes good." This to me, was terribly selfish and on my birthday (November 8) I ordered some ribs and decided that would be my last meal with meat on it.
So this preachy kid I mentioned (who by the way eats meat now, which is kinda funny) was not just a vegetarian, he was a vegan. But what could possibly be wrong with milk, eggs and byproducts? I didn't know. I did some research on my own, and though it may have been a little skewed by organizations like PETA putting spins on things, I did find out how the mass production dairy farms work here. And it all makes sense if you don't give a FUCK about the cows. Treat them like machines, pump them full of hormones and take their calves away and ship them off to veal farms. This all seemed kinda fucked and I realized that free range stuff wouldn't cut it for me because it was too pricey and I would probably just end up cheating whenever I wasn't somewhere with access to free range stuff, so I just cut it all out. I decided at that point that there was no reason to take part in animal products besides selfish convenience and therefore I HAD to stop.
Since then I have learned about the effects of meat, dairy, etc on the environment. Veganism just makes sense to me. That is all there is to it. Don't harm something that cares if it is harmed, and animals do care, if you can avoid it.
I did not mean for this to be too preachy (though if it makes some of you think, that isn't really a bad thing), but this is just a place I can send people for my story instead of having to write it out a million times like I always do.