I know a few vegetarians. One is for health reasons – his body can’t digest the meat properly. I have no problems with such a person, because your health comes first. If he was able to handle it, he’d love to eat meat – he always complains about not being able to.
Another person I knew says she doesn’t like the taste of most meats. If it’s ground up in something, and absorbs another flavor, she’ll eat it, and she’ll eat around meat – so on Shabbos, she’ll have cholent, just not the big chunks of meat that might be in it.
Then I know people who do it because they’re stupid liberals. I argue with them, and try to convince them that they’re wrong. My wife got upset at me for not treating guests properly – she thinks I shouldn’t argue it with them.