To be quite honest, you shouldn't have to come up with a reason you date inter-racially. The person who asked you that question should have to explain why they asked it, or why they don't. There is no reason NOT to. I understand that some people seem to have this idea that you should stick to your own race, but in all honesty, I don't think there is one person on this planet who would exist as themselves if everyone had ever just stuck to their own race.
And if we want to get technical, you ARE dating your own race, given that I assume they are all human, right? ;-)