I, too, have been asked this question and been stumped.  Since it's been
discussed here, I've spent a bunch of time thinking about it, and I might
have the answer, well, OK, I have two possibilities:
 
1. Because they have so much personality and joy in life, you can't
   help it.
2. Because they demand it. :)  (You don't think so?  Try ignoring one
   for a little while.  LOL)
 
Debbie
[Posted in FML issue 4859]