As the life sciences evolve, theories of unidirectional effects often get replaced by theories involving cyclic or reciprocal effects. One area where effects have been considered unidirectional until now is trait theory. In fact, the Big Five are generally posited to be stable, and when change is discussed, a trait is considered as either an agent of change or a patient of change, but never both simultaneously. With regard to well-being, traits are almost always treated as agents of change. We recommend a revision to this model, by putting forth a reciprocal model of causality between well-being and traits. Using panel data from the Midlife in the United States random digit dialing sample of adults (N = 1602), we show that there is a reciprocal loop between psychological well-being and the Big Five, with extraversion and neuroticism being the most important factors on the trait side. These data call into question the set point theory of well-being, and they indicate that Fredrickson’s broaden-and-build mechanism of positive affect operates on larger time scales. The notable role of extraversion and neuroticism suggest that the attachment–avoidance system predicts exposure to positive vs. negative life events, and this system is in turn modulated by exposure to positive vs. negative events.