The Evolutionary Explanation

Adaptation probably evolved because perpetual satisfaction would be dangerous. An ancestor who felt permanently content after one successful hunt would stop hunting and starve. The brain resets satisfaction toward zero so you keep striving.

Your brain is not designed to make you happy. It is designed to make you survive and reproduce. Happiness is a temporary reward signal, not a permanent state. Each spike of pleasure is the brain saying "do that again," not "you have arrived."