It is very interesting that exercise influence the reward system by increasing dopamine release in VTA and NAc. It may be similar to drug, food, sex and gambling which increase dopamine transmission and lead to addiction but the severity of addiction may be different which depend on tolerance (diminish response to the drug etc.). As we know, the exercise is also increasing endorphin that correlate to your results which increase dopamine level in reward system. Definitely, the increasing of dopamine from exercise may cause a pleasure emotion and motivate to reward by do the same exercise every day. Some question is required to answer are how exercise effect on the increasing dopamine and this effect lead to addiction and tolerance effect like drug. So, the molecular mechanism is need to explore and this may answer how to drive the workout behavior in the molecular aspect of exercise.

I agree with previous responses on this forum. The issue is addressed well.