People have free will, which is subjective judgment, personal preference and choice. Psychological studies show that the more people work hard to achieve success, the happier they feel; The more reflective things are, the happier they are; The more self-controlled the satisfaction is, the happier it is. Psychologists have tracked that children who tolerate their urges more often as adults are more likely to get into college, get married and earn PHDS, while those who resist immediate gratification are more likely to drop out, divorce and suffer more frustration.
The text's main idea is that the degree of control over free will determines how happy people are.