Speaking as someone whose been a vegetarian for over a decade, I sometimes am off-put by others being any form of veg. It isn't that I want to feel special or anything like that, it is that in my age group (I'm 19, nearly 20) there are loads of people doing it because it is the "in" thing right now. They may respect animals more and the environment, but a lot tend to only last a year or two, then go back to eating meat. I've been seeing this since about the beginning of high school. When adults (like, over thirty-five) say they are, I have much more respect as they tend to have been so for a very long time and it is their lifestyle choice, not a trending statement to drop in a few months only to pick something else up.
I think part of my issue is that I've been veg so long that I don't remember what meat tastes like, the smell of it (or even just overly greasy and fried food in general) makes me stick to my stomach, that I don't really have much of an understanding of it being hard to make the choice. I was seven when my parents gave me the ultimatum to either eat meat or go veg, them expecting me to eat meat, but even then they respected my decision and I haven't looked back since. Also, when I was a kid, I had bad experiences of food poisoning from meat too often so by the time I did become veg, the only meat I was eating was lean chicken (poultry, fine), so I wasn't really missing anything. I never had a rocky start with being a vegetarian since I'd already disliked chicken, so I've never felt any temptation to stray from my lifestyle.
Once I get to know people who are veg, pes, or vegan, though, and I can judge there character a bit more, I tend to be able to pick out who actually cares and will care in the future, and who says they care but are really part of the trend (even if they don't even realise they are - many of my friends think they are "counterculture" by becoming it, when they really are just counter to one culture, but fitting into a trend of another).
As for the reasons why do it, I understand those who do it for diet. I probably would have eventually done it for diet if I continued eating chicken since I wasn't able to eat beef or pork ever - never worked with my stomach - and I disliked seafood. Others doing it for health reasons I also respect. Basically any reason except it being a trend and I'm pretty much will respect you. (It may have become clear I have a bit of a pet peeve for those who do it because it is "in" and whatnot.)
Also, adding this in, I dislike self-righteous vegans and vegetarians. They will ***** about being the only ones truly caring for animals - right. Most of you have only recently, as in the past five years, become vegan. You want to go into numbers? I haven't had meat for about thirteen years. Before that I had chicken maybe once a week, if that. Tried beef twice, pork once, shrimp once, and salmon once. That's it. I get non-fertilized eggs from cage-free, vegetarian hens from a local farm - I literally pick my eggs out. It is a small farm that doesn't have run-offs. If I get milk (which is rare), I also get it from them. They are nice, good people. They care for their animals and are a vegetarian, elderly couple. There isn't a leather product to my name. I don't smoke cigarettes, which means that even if you have a cigarette that is allegedly not from animal testing, they got product information from another cig company that did. You can take your self-righteousness elsewhere, because you are what is wrong with the veg community and who deters interested people away from it.