What is the fundamental reason why people lack confidence?

Scientists observe lobsters fighting.
After a fight, the victorious lobster secretes more serotonin, making it more arrogant and less likely to back down when challenged. It appears tall and threatening, easily scaring off other competitors. The winning lobster gets the best shelter and food sources, which makes it stronger over time.
The defeated lobster, no matter how brave before the fight, will lose its fighting spirit afterward and may even be unwilling to battle former rivals. Failure destroys its confidence, and this state can last a long time. If a once-dominant lobster is defeated, it will secrete less serotonin, leading to brain restructuring to adapt to its new humble position.
The difference in outcomes of early battles is subtle, but it gradually widens:
The winner is a king lobster, flaunting its dominance; the loser is a shrimp, bowing its head in submission.
Biologically, humans are roughly similar to lobsters.
If a person keeps failing, they will lose confidence. The less confidence they have, the more likely they are to fail again—a vicious cycle.
In this way, it creates a negative feedback loop, and any living organism will become dejected.
Therefore, success is the mother of success; failure is not the mother of success. The more you fail, the more you fail.
Winning once might just start a positive cycle!