When a mammal meets another mammal, each brain instantly decides whether or not it can dominate. Survival depends on it.
Why can't we all be equal, you say?
Equality is an abstraction you create with your cortex. Your mammal brain just wants to know if it can grab a piece of food without getting bitten by the mammal next to it. A mammal starves if it never goes for the food in front of it. But it if takes something a bigger critter has its eye on, it is quickly bitten or scratched. Painful experience teaches animals to make a submission gesture the instant they see a more dominant individual.
Of course we're not animals. But we've inherited the limbic system that mammals evolved to survive in groups. This limbic system controls the neurochemicals that make you feel good or bad. If you want to feel good, you have to make peace with your limbic system.
The mammal brain releases serotonin when it's in the one-up position because that promotes survival in the state of nature. The serotonin feels good, which motivates a mammal to seek the one-up position again.