~humans are the only animal that continue to drink milk after weaned of their mother's
~growth hormones that are injected into cows and chickens are then consumed by humans
~they just don't like milk or meat
~they have allergies
~the idea of eating the result of a chickens period is not pleasant
~those appetizing limp greasy Whoppers don't appeal to them
And more. People, who feel guilty for eating meat but are too weak to give it up, often refer to them as Nazi's. However, most vegans don't think down upon people that eat meat.
Herbivore: No, I'm vegan.
Omnivore: Oh, so you think I am a meat eating pig who enjoys animal cruelty, just a jerk who desrves to die because I won't chain myself to a tree to save a forest?
Herbivore: That is just a common misconception. I don't think that way about anyone except for bush. Let's go to the movies.
Omnivore: Sounds good!
Also not inherently self-righteous...but altogether too many of them are.