Or maybe I'm not. It probably depends on who you ask.
Last night, I caught this joke (and I use that term lightly) going around Tumblr that said something like "Want to know who the vegan is at your party? Don't worry, they'll tell you." Essentially playing into the annoying stereotype of the preachy PETA-loving vegan who can't talk about anything else.
The stereotype is actually far more annoying than any vegetarian or vegan (from here on out I'll combine the terms into veg*n) I've ever met.
See, here's the thing. People don't adopt a relatively extreme lifestyle like veg*nism unless they either have to or feel passionate about it (though I will say vegetarianism isn't really THAT extreme). Let's even erase the "have to" part of things and focus on the veg*ns that do it because they feel strongly about animals.
When you're at a party, talking to people, what do you discuss if it's up to you? That's right, things you are passionate about. Whatever the topic -- football, church, gay rights, veg*nism -- why would you bring something up that you aren't interested in? And when it's a topic that's always near the surface (you have to think about it every time you eat) it may very well be the first thing that comes to mind when you're scrambling for a topic.
There's another scenario that I've encountered before. Sometimes you mention being veg*n for some reason, usually in passing, and it gets picked up by someone in the group you're taking with and it snowballs into a debate about the issue that you didn't even want.
It's for that last reason that I put forth the idea that there are far more sanctimonious meat eaters than veg*ns. And what's worse, the obnoxious non-veggies think they're being funny when they do it.
I of course am not saying all meat eaters are like that. Unlike the "joke" above, I am not stupid enough to generalize an entire group of people based on their dietary choices. I am, however, going to point out that I have met absolutely no sanctimonious veg*ns, but many people who claim all veg*ns are irritating and seek to make fun of them.
I guess since I occasionally mention (in conversations and in food settings where it's relevant and important) my vegetarianism and answer questions people ask about it, I'm obnoxious. If being open about my choices and standing up for myself is sanctimonious, then that's what I am. I'd rather be that than the kind of person who perpetuates stereotypes about other groups.