Master told me that before he got to know how masochists (me, and others) think, he always had assumed that S&M was about the Top's pleasure, and the only thing the bottom got out of it was the pleasure of having submitted and pleased their Top.
I had always thought the opposite, before learning more about sadists. I always thought the majority of pleasure must being going to the one on the bottom.
It is funny how this perspective was in place before we even started doing this ourselves.
Now, I think we both rationally, on the surface, understand that both sides are getting something good out of it, but what that something is, I'm not sure either of us can really understand at a deep level.