First of all, Dave will never be vegan. He loves cheese too much, and cheese is a crucial ingredient in pizza. But cows don’t die in the cheese-making process (that we know of) so Dave can rest easy with his dairy habit.
Dave has not eaten meat in almost year. Go Dave! He’s a vegetarian not because he’s grossed out by red meat like some picky 16 year old girl on a diet. He also isn’t too phased by the dangers of artificial growth hormones or antibiotics in most meat, either.
It’s something to do with non-violence. That’s also why he doesn’t kill cockroaches.