I know one vegetarian (my sister) and I believe she does know her objection. When we were young (10-11) she stopped eating meat after visiting a farm. As far as I know, she's never eaten meat since then. She says she just had the insight that animals were "real" and didn't want to eat them anymore.
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous "stupid" questions thread is at almost 500 questions in about a month, so I think it's time for a new one.
Also, I have a new "stupid" question.