What makes me happy about my bike..
Riding it ...
The Clunk, clunk, clunk of a well adjusted derailleur as it shifts down as I build speed.
The Chik chik chik as a well adjusted derailleur moves up the cassette .
I have to admit to some sort of strange thrill from coming down a steep hill (plenty to chose from around here) choosing a point to stop by, usually a lampost or sign, sprinting hard towards 40mph then braking as late as possible and trying to stop spot on at the side of the target without easing of the brakes, It seems to keep my disk brakes in good order as it pumps some heat through them. Through it is probably good for nothing.
Oh and climbing hills, or rather getting to the top of hills.