Summary of “The Real Roots of Midlife Crisis”

Part of the answer likely involves what researchers call selection bias: unhappier people tend to die sooner, removing themselves from the sample.
A common hypothesis, and one that seems right to me, is alluded to by Carstensen and her colleagues in their 2011 paper: “As people age and time horizons grow shorter,” they write, “People invest in what is most important, typically meaningful relationships, and derive increasingly greater satisfaction from these investments.” Midlife is, for many people, a time of recalibration, when they begin to evaluate their lives less in terms of social competition and more in terms of social connectedness.
In my 40s, I found I was obsessively comparing my life with other people’s: scoring and judging myself, and counting up the ways in which I had fallen behind in a race.
Carstensen told me, “When the future becomes less distant, more constrained, people focus on the present, and we think that’s better for emotional experience. The goals that are chronically activated in old age are ones about meaning and savoring and living for the moment.” These are exactly the changes that K. and others in my own informal research sample reported.
“As people perceive the future as increasingly constrained, they set goals that are more realistic and easy to pursue.” For me, the expectation of scaling ever greater heights has faded, and with it my sense of disappointment and failure.
He used a German longitudinal survey, with data from 1991 to 2004, that, unusually, asked people about both their current life satisfaction and their expected satisfaction five years hence.
To his own surprise, he found the same result regardless of respondents’ economic status, generation, and even whether they lived in western or eastern Germany: younger people consistently and markedly overestimated how satisfied they would be five years later, while older people underestimated future satisfaction.
What’s more, Schwandt found that in between those two periods, during middle age, people experienced a sort of double whammy: satisfaction with life was declining, but expectations were also by then declining.

The orginal article.