According to the American neuropsychologist Damasio, willpower mainly stems from self-preservation. A person becomes motivated to do something well if it benefits him. Someone is going to do something, if he somehow gets a reward for it. In an evolutionary sense if it increases its chances of surviving in the most pleasant conditions. Damasio goes so far as to attribute altruistic behavior or empathic behavior to this somewhat selfish drive. Empathy presupposes reciprocity. If someone adopts an empathetic attitude, he may also expect empathy in return. Of course it also works the other way around. A person will not do something quickly if he is punished for it. In reality, however, it is more complex. There are many situations imaginable in which someone is punished in the short term, but has a reward in the long term. That reward may be more valuable than the damage of the punishment.
For example, it could be a student who gives up a night out with his friends to better prepare for an exam. But also a resistance fighter can risk a very serious punishment or even death, because the reward in the long term, a better society, but also in the short term, the feeling of acting like a good person, serves as his motivation.
Does that make altruism or empathy lesser feelings? Not at all. It is precisely these mechanisms that ensure that many more good and pleasant things happen between people than bad things and that we morally reject bad things.

Adolescents are highly focused on short-term rewards and especially social rewards from peers. For adolescents, a reward is more important than for adults. In fact, social rewards are so important that adolescents, especially boys, are willing to take a lot of risks in the presence of friends. They do not take those risks because they are not yet able to properly assess the dangers. They know well that a certain action is dangerous. They accept the danger because they want the reward so badly. In addition, they are less able than adults to slow down or stop behavior once started.
Evolutionarily, adolescents are strongly peer-oriented. It is better to invest in the future than in the past. They ultimately have to prove themselves in the world of their peers and not in the world of their parents.
Moreover, brain scans show that the inverse of a social reward, namely being excluded by peers, elicits the same reactions as endangering one's health or cutting off a food supply. Being rejected is a life-threatening situation for the brain.

Explicit learning is mainly aimed at long-term reward. You learn at school to get a good job, to make a career and to be successful in adult life. Those are things that keep teenagers busy. However, if at a certain moment the choice has to be made between this distant prospect and a short-term reward, the choice often falls on the latter.
Rewards are not the only motivators that move adolescents. They are also highly curious and sensitive to new experiences. Their brains are hypersensitive to neurotransmitters such as dopamine and oxytocin. Dopamine not only plays an important role in activating the reward system, but also in learning patterns and making decisions. Oxytocin ensures that social contacts are experienced as pleasant. The neural networks involved in rewards and social interaction overlap and reinforce each other. As a result, adolescents can react very violently, both when something succeeds and when something fails. In other words, both when they receive a reward and when a reward is withheld.