I've flirted (in thought, only) with vegetarianism, but have always arrived at the conclusion that those things which are able to eat me would, if given the chance, and those which are unable to eat me would, if given the ability.
I have a whole weird cosmology of mostly mystic woo-woo reasons for most of what I don't eat:
Cows and chickens: Painfully stupid, sure, and chickens are apparently also plenty mean to boot, but it's all our fault since we have been systematically breeding them for maximum stupidity and helplessness for generations, and the thought of supporting the whole craxy industry by eating them makes the "humans should be the careful stewards of creation" part of my conscience cringe. Plus, freak-ass statistics like "XXX gallons of water to make a single hamburger" and "XXX hundreds of pounds of grain to make a single steak" and all that.
Wild birds: We've managed to not domesticate them into lolling stuporous masses of waddling cholesterol; can't we just leave them alone?
Veal and lamb: BABY MAMMALS. With the BIG BLINKY EYES and the EYELASHES. BABY MAMMALS.
Rabbits and other small mammalian game: Rabbits might eat humans if they could, but absent-mindedly, not viciously, as a rabbit's chief joy in life besides making more rabbits is chewing on stuff. Gardens, table legs, high-speed internet cables, books, boxes, you name it. They're not malicious about it, they're just madly in love with chewing. Whereas I never chew on anything absentmindedly; I chew with deliberate intent and usually much forethought and anticipation. So chewing on rabbits in a way they'd never chew on me makes me feel guilty. Plus, had a rabbit as a pet and he was the cutest fucking thing in the history of cuteness.
Fish: We're wrecking the oceans and rivers and we've cleverly managed to load up anything that lives longer than an oyster or a scallop with lots of yummy mercury, so they're not only poison, but we basically don't deserve to eat them.
Cephalopods: See above. I don't eat them often, in case their edibility and deliciousness with a nice garlic aioli is just a ruse to get me good and poisoned, but when I do, I eat them remorselessly.
This is why I have never, ever tried to convert a single soul to vegetarianism, because my own brand of it is simultaneously so very splendidly half-assed AND crack-addled.