I used to be one of those people who thought vegans and vegetarians were a bit silly/overly sensitive/whatever. I grew up on a small family farm where we raised much of our own food, including beef, lamb, chicken, eggs, and milk. I ate animal products all through college, and even went paleo for about two years.
What lead to the shift to veganism for me was working on an organic, small family run chicken farm that offered supposedly pasture-raised chickens. From their website and photos, you would have thought that this was a great example of humane and healthy farming. What I saw (and breathed) while I worked was enough to put me off chicken- mostly from disgust, but there was some glimmer of ethics there too. This farm happened to be well known at the time, occasionally touted by local restaurants as their sustainable and humane meat source. I admit that it's an extrapolation, but I figure, if they they could charge $6/lb for whole chickens and the chickens in the grocery store are $.99/lb... well, I'm going to assume that even accounting for economies of scale, that $.99 chicken came from worse conditions.
The next step was eating vegan for 30 days, which I did primarily to challenge my prejudices against vegans and vegetarians. Whlep, we all how that turned out! My asthma, which had always been mild but nevertheless a rather annoying limitation, went away, my kitchen was so much easier to clean, and the grocery bill went down a significant amount as well. After those thirty days, I didn't want to go back to eating animals. A few more months of eating vegan passed, and I realized that while I'd always espoused the opinion that if you're going to eat meat, you should be willing to kill it yourself, the only time that I'd actually been able to kill anything was to put it out of misery. A few more months went by, and I realized that I don't value meat enough to kill for it.