Pet Theory about Exercise, Dopamine and Motivation

I've read from several sources that exercise is good for depression, for giving you more energy, focus, etc. Several recent studies support this idea. Unfortunately, there doesn't seem to be that much of an explanation available as to why this is, other than "exercise is good for you". Now, I'm not a neuroscientist, merely a computer scientist who's interested in the brain. As such, I'll acknowledge that my theories might be approaching the realm of "broscience". I still believe these thoughts are worth sharing.

Dopamine has many roles in the brain, but one of them is linked to motivation and energy. It seems that dopamine is involved in "gating" outputs in the motor cortex. Your brain can simultaneously produce many possible ideas of movements or sequences of movements that you could make, some more likely than others. Higher levels of dopamine lower the threshold required for these movements to be executed. Higher levels of dopamine literally make it so you need less willpower to execute a movement, to get out of bed and work out. If your levels of dopamine are low, you will only execute movement ideas that your brain makes extremely salient. For example, when you're hungry, t…