It depends on who they are and why they are asking. I usually give a general answer, like “For reasons having to do with socio-economic and environmental justice,” or “Because i read Diet for a Small Planet when i was ten.”
Most people aren’t looking for a conversion spiel, and will be put off if you try to give one. So i try to be non-confrontational, answer quickly, and they can ask for more information if they want it.

I don’t get asked that often, probably because i live in Portland, OR, and it’s not that unusual. But really, just think of your response strategically (assuming you do it as a piece of integrating activism into your life). If it’s not going to make any impact on the hearer, don’t put a lot of effort into it. Answer quickly and move on.