Meh, pretty thin discussion of midlife crisis. And focusing on "happiness" isn't a great way to measure anything.
It seems the midlife crisis comes about because many people "hit their stride" in middle age. All they've known to that point is struggle.
By middle age, the kids are mostly grown (or at least self-sufficient for managing their day and don't "need" mom and dad like toddlers do), you know yourself pretty well, work has become less challenging (it's wash/rinse/repeat).
You've been pushing against a wall for decades, now the wall isn't there, or it's less important to you, and you don't know what to do now - there's a loss of meaning because of this change in perspective.
Meaning in life comes from such things - facing challenges. Without the challenges to which one becomes accustomed, people are adrift for a bit.