Why are smart people in 30s unhappy? And what to do about it

Early 30s can be a strange phase for the reasonably successful - good money, great partner, international vacations, nice car, all boxes ticked, yet everyone seems unhappy. Why? After a few years, the challenges at the job wanes, but the perks don't let them leave the comfort zone. Absence of challenges makes the journey perpetually boring, and causes unhappiness.