It's a matter of integrity.

I also talk about basic integrity when people ask me why I’m a vegan: I don’t want to pay others to do things to animals that I wouldn’t want to do myself. Everyone agrees that cruelty to animals is a bad thing. Most people will express remorse at the horrible things that are done to farmed animals on factory farms and at slaughter. So the big question is, “Why pay people to do things that you don’t support?” In explaining his vegetarianism, Percy Bysshe Shelley said that he wants no part of anything he can’t write a pleasant poem about; we shouldn’t be supporting things that revolt us. How many things are there in our lives that we’re directly supporting, even as they revolt us? Of course, all of us could spend an afternoon picking grains, beans, fruits, or vegetables, but who among us would want, even once, to rip the testicles from a pig’s scrotum without painkillers or to sear the beak off a tiny chick with a hot blade? Who among us would want to even watch any aspect of what is required to get chickens, fish, pigs, cattle, dairy products, or eggs to the table? Who would want to spend even five minutes in a slaughterhouse, with all the blood and horror? Raising this issue with people in a conversational way can help them to see that eating meat is ethically dubious—it’s paying others to do things that they don’t support or believe in.