Why is sugar on top of sugar so much better?

It's so funny when you take a cookie packed with sugar and butter, and you think how can I make this better?  Oh wait I know, just add frosting.  It's just funny that something so cramed with sugar would need any more sugar to tame the tast buds of a cookie-holic.

Funny how we are, the sweeter the better.  But can we over do it?  I mean can a treat ever be to sweet?  I guess that is to be proven.