For me, an almost-always vegan who eats cheese or meat in social settings, it was a general despair that the moral argument for veganism is so simple. It's easy to intellectually coast in this area for a while, because all the arguments meat-eaters present in favor of eating meat are so astonishingly and universally dumb - we evolved to eat meat, meat gives me pleasure (ignoring consequences of that maxim), some distinction between humans and animals which falls apart under scrutiny or has consequences for treatment of the mentally disabled, etc. I've seen even very intelligent people put these forward. Converting from meat-eating to veganism carries with it a component of acknowledging an underlying personally-applicable horror in the world which is similar, I would say, to becoming atheist. It's thus nearly impossible to reason with reasonable people here, simply because the psychological consequence of losing the argument is so strong. PETA recognizes this, which is why their videos focus on the graphic ugliness of the meat industry to provoke an emotional reaction.

Anyway. I originally really liked the moral arguments for veganism as forwarded by Peter Singer, which of course is very similar to his argument in Famine, Affluence, and Morality - it's pretty much impossible to accept one but not the other. I have more problems with the consequences of the FAM argument - that we inexorably ought to use all our life for the benefit of others if we profess to care for others at all. This, to me, is such an absurd consequence as to throw the entire enterprise of morality as a chain of material implication following from axioms (very attractive to a computer scientist!) into doubt. Which, I've been told, casts me out into the great despair of postmodernism where we don't have access to any meaning or oughts at all. I'd love to talk about this with someone who knows more about it, since my philosophical education pretty much comprises a single ethics course in university, some existentialist literature, some SEP reading, and a whole lot of PEL episodes.