No, we don't - we have a biological framework for creating motivation for certain behaviors. This is a well established concept. Motivated behaviors (or habits) are the framework, addiction is when we misuse that framework. We're not born with some defect that gives us negative evolutionary fitness. That makes no sense at all.
yeah, we do. and it involves neurotransmitters.
"addiction" may not be the right word in this (workout) context, but it's a well established concept in other contexts.