Young adults typically believe that they’ll “beat the average” — that they’ll be the lucky ones who end up with a top job, a happy marriage, and healthy children. Neuroscientists believe that overoptimism is based on biased information processing in the brain, which makes it difficult to correct overly rosy expectations in the young (this bias might actually be evolutionary efficient, as a powerful driver for seeking progress).
As we age, things often don’t turn out as nicely as we planned. We may not climb up the career ladder as quickly as we wished. Or we do, only to find that prestige and a high income are not as satisfying as we expected them to be. At the same time, high expectations about the future adjust downwards. Midlife essentially becomes a time of double misery, made up of disappointments and evaporating aspirations. Paradoxically, those who objectively have the least reason to complain (e.g. if they have a desirable job) often suffer most. They feel ungrateful and disappointed with themselves particularly because their discontent seems so unjustified – which creates a potentially vicious circle.
It’s at the bottom of the U-curve (the mid-50s, according to the German data) when expected life satisfaction aligns with current satisfaction levels. People come to terms with how their life is playing out. At the same time, the aging brain learns to feel less regret about missed chances, as brain studies have shown. This combination of accepting life and feeling less regret about the past is what makes life satisfaction increase again. And since people over 50 tend to underestimate their future satisfaction, these increases come as an unexpected pleasant surprise, which further raises satisfaction levels.
https://hbr.org/2015/04/why-so-many-of-us-experience-a-midlife-crisis
Know Thyself 