Short Answer :
Lucky Go Happy puts forward the theory that our haves, and our wants are out of balance as we move through life. As an example, when we are young, we have youth, but want to be grown-up (the future). When we are old, we finally have that future, but then we want to be young again.
A graph shows that the curve for our wants, intersects with the curve of our needs around the middle of our lives. Mathematically speaking, we therefore have what we want, and want what we have; albeit in moderation and this results in a midlife crisis.
Further Reading :
Chapter 22, specifically the simple graphs explaining the predicament of life in