What is the fundamental reason why people lack confidence?

Scientists observe lobsters fighting; after a victory, the victorious lobster secretes more serotonin, making the lobster more arrogant.
When facing challenges, it is also unlikely to retreat, appearing tall and threatening, easily scaring off other competitors.
The victorious lobster gets the best shelter and food sources, which makes its body stronger over time.
The defeated lobster, no matter how brave before the fight, will lose fighting spirit afterward, and may even be unwilling to fight former rivals.
Failure destroys its confidence completely, and this state can last a long time.
If a once dominant lobster suffers a defeat, it will secrete less serotonin, leading to brain restructuring to adapt to its new humble position.
The difference in wins and losses in early battles was subtle, but afterward it gradually widens: winners become dragons, flaunting their dominance; losers become shrimps, bowing their heads 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, creating a vicious cycle.
In this way, it results in a full-body negative feedback loop, and every organism becomes 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 open the door to a positive cycle.