Motivation is expanding resources to get a reward.
What is a reward?
Dopamine is not reward.
Dopamine is effort.
Dopamine is necessary for effort, not reward.
People can find things rewarding without dopamine but they need dopamine to move, to encode information. It is effort required to work towards getting a reward.
Reward is bringing the body back into a low statis. It feel good when that happens. People will work tremendously hard to have that happen.
Serotonin is a neurotransmitter that allows your brain to delay gratification of a reward. It allows you to expand energy now, because you anticipate a reward at some point in the future. If you have a deficit in serotonin, you can’t do that very well.