"Why are you a vegetarian?" What is a good answer to this?

I have a lot of reasons for being vegetarian, and I think I say something different every time somebody asks me. Like you, I never know exactly how to answer. Since it's a very personal decision I don't like to go into the details for just anybody. I usually say something like "for my health" or "for ethical reasons." In your case you could say that vegetarianism is better for the economy. If somebody is offended or defensive over those responses, there is no help for them!