It's interesting how such a short question led to such a long thought process. And after going through myriad excuses/rationalizations/reasons, it all boiled down to these:
Because I love to hear myself talk.
Because I love to see the lightbulb go on over someone's head when they "get it."
Because I have a gift that would be wasted if not shared.
Because if I didn't, no one would remember me after I've died.