Slumps are commonly attributed to psychological weakness, loss of motivation, or declining ability. In this paper, we present a structural and dynamical theory of slumps based on a success index model. We show that a slump is not caused by a decrease in ability, but by a rapid increase in expectations that outpaces the growth rate of ability. By decomposing expectations into social and self-generated components, we derive necessary and sufficient conditions for slump emergence, recovery, and long- term success maintenance. The analysis reveals that humility and exposure control are not moral virtues but optimal control strategies.