Why? Why Not?

I don't remember when we started playing this game. I think it could be about a year now. My son just turned two. The first time he asked me "Why?" I answered his question. I didn't hear it again for a while, but then it became more and more frequent. At first it didn't seem like he understood what he was asking just by looking at his face, but I would answer him anyway. He'd keep asking and I'd keep answering. We play the never-ending "Why?" questions/answers game.

Now it has progressed to "Why not?" and sometimes he would add a string of words after that. It seems forever ago, but when he first asked me "Why?" I almost didn't answer him. I'm glad I did, I think that it has helped develop his language skills. The thing is now I am telling what he cannot do? And of course he will come back and ask "Why not?"

Sometimes, I can't think of anything better to say but "Because I said so" or "Because I asked you to." Does anyone have a good answer for me.