Judging.

It never fails to amaze me how judged some people can feel by other people's choices. I'll preface this by saying that I am guilty of it as well.
But when I tell people I'm a vegetarian, the reactions are one of three:
1) "I used to be a vegetarian, but then X happened and now I'm not."
2) "I would be a vegetarian, but I think it's too hard/meat is delicious/I hate vegetables."
3) "You shouldn't be a vegetarian - you won't get enough protein/meat is delicious"
I don't care why you're a vegetarian, and you shouldn't care why I am. It's okay to ask why. It's not okay to tell me that I'm going to die sooner because I don't eat enough protein or that humans are meant to eat meat. You are actually the one who will die sooner, because you eat too much protein and humans aren't meant to eat meat. But I don't tell you that when you order a hamburger so let me eat my veggie burger in peace.