So this would be an addiction that we benefit from.
Sounds good. Here is where an evolutionary biologist would chime in and say it gives us a competitive advantage in natural selection and may be the very reason that exercise 'evolved' as one of the triggers on the pleasure center.

What do you think?

Seems pretty easy to figure out....
I've always said fit girls tend to be cute. Makes sense that fit dudes are cute to women.

Cute dudes and cute girls make babies, genes are propagated, the rest is history.

Eh, I just see it as a manifestation of compulsive behavior in the exercise-aholics I know. A better outlet for it than most, to be sure, but they would (and/or formerly were) just channeling that same basic personality trait into some other activity otherwise... like collecting, or being an obsessed fan of a sports team, or maybe even something their kids were involved in, whatever...