When people have been asking me why I shaved it I've been telling them it was inevitable anyways and why wait till I look rediculous before doing something about it.
I always say that I just finished what mother nature started.
Funny how we all wind up at the same place, I've used those for the few that have asked and lately have added, "Being bald is great, going bald is the SH_TS."
When someone asks me why I shaved my head, I just tell them, "Because I felt like it..."
Most people stop there. If they don't, and ask another question, I consider that they may be truly interested, and we usually have a nice conversation.