We all generally go through some stages of life when we have slightly different abilities, perspectives and goals. The simplest is when we are babies and only have basic needs and thoughts,,, another stage of change is puberty, then we have the age that we reach legal adulthood. As young adults we are looking forward to establishing careers and families.
Then, when we reach middle age we look both forward and back. We look back at what we have accomplished. We think about what things we failed to accomplish , and we take stock. We look forward at the prospect of declining abilities. We often decide we have one last chance to try to accomplish the things we feel we lack before we get 'old'.
So,, men, and women, then hang on to a youthful image, maybe have a fling,, change jobs. The significant others , who may see this person differently, may not understand why s/he feels that they lack anything, or have need to have any flings.
The middle age crisis usually resolves when the person accepts where his or her life is at and settles back down,, with new or old job, new or same spouse, and feels ready to proceed with life at a more 'mature' level.