New research suggests why:
"A new working paper by Hannes Schwandt of Princeton University proposes an intriguing possibility: the gloom of middle-age is what happens when high expectations are dashed. Using data from a large survey of Germans, Schwandt contrasts how, for example, a 25-year-old expects to feel at 30, with how she actually feels when 30 comes along. The pattern is striking: young people have vastly inflated expectations of their lives five years on. Expectations ebb as the years go by, although it is not until their early sixties that people start being pleasantly surprised by how life has turned out, relative to expectations five years previously."
Sounds an awful lot like the second noble truth of Buddhism.