There are multiple compounds that effect motivational energy, because that behavior is a consequence of how our brain functions. So things that modify the related circuits will modify the behavior. Plus free will is an illusion, and we don't actually "choose" anything. All of our behaviors are a consequence of the biochemical functioning of our cellular circuits. Quite frankly, the GP has no idea what they are talking about.

If you want to start understanding motivation and reward, simply look at the function of mu-opioid agonism in relation to the NAcc and related dopamine task reward circuitry.

Absolutely, Bupropion(Wellbutrin) acts as a norepinephrine–dopamine reuptake inhibitor and a nicotinic receptor antagonist.