> I recently thought that pain is very certain and reliable: stub my toes and it’s equally painful every time.

Mental state matters here. A cut stings, but I've been cut and not noticed.

> Pleasure is also consistent but less so: eating an applie pie is pleasurable. Maybe not two days in a row though.

I think some forms of pleasure stem from novelty. "Variety is the spice of life" and all that.