This is something I have been thinking about a lot, recently. I have a friend who has gone vegan in the past year, and when he first talked to me about it, he had what I felt was a refreshing attitude towards it... he talked about it him doing what felt right for himself, and said that he felt he'd make a more positive difference to the world by having a positive attitude towards his and other peoples' choices, and that people were more likely to be open to the idea of a vegan lifestyle having met open and non-judgemental vegans rather than the more militant type.
Recently, though, it's all gone a bit more "you're a bad person because you eat eggs", which made me sad. And he happily says he looks down on people who eat meat. He's very keen on converting people... though really, you catch more flies with honey than with vinegar, right?
I've been a vegetarian for seven years now - was vegan for a couple of months about five years ago but am more comfortable as a vegetarian. Something I have noticed, is that a lot (not all, of course) of vegans I know are very condescending and occasionally aggressive towards me, and they often say things to me like "...but by not eating meat, that's a really good start" *cue angelic music and halo around their head*
...veggies do it too, of course... and there's always something to look down on other people about: eating things containing egg when you don't whether those eggs are free-range, buying from The Body Shop when they are now owned by Loreal, taking gelatine capsules...
Why? Why not respect other peoples' choices, and be supportive? We're all on the same side, right?