Your reward system is a fragile thing that is easily broken. Just look around you next time you go anywhere. We live in a society of smart phone addicts. That said, the examples OP gave of tunes while exercising seem pretty benign compared to superstimuli like social media or drugs...

We are in agreement. When I say it's not fragile, I mean it takes more to mess with your reward system, like using your phone all day. It does not take listening to music for 20 minutes.