[Fnoord]

There are three global reasons one can decide to be vegan, vegetarian, and reducetarian.

1) Animal well-being.

2) Environmental well-being.

3) Personal health well-being.

You may substitute well-being with suffering, and there's also a nuance between death and staying alive but using (e.g. milking).

[EDIT: I suppose there's a fourth, a psychological reason how some people find certain products 'impure' to eat. But I'm not sure how I'd define that group.]