So many people are unhappy because they don't believe their life is turning out the way they'd planned it. They had hopes and dreams and visions of grandeur that are slowly fading into the reality of day to day life.
Midlife crises occur when people stop and take stock of their life and realize that it is never going to be what they had imagined - they will never achieve all the goals they had envisaged. They will never be as rich, or famous or whatever as they'd assumed they'd be by this point in time.
Maybe there is a different type of midlife crisis - one where you stop and think to yourself how good life is despite not being the life you had planned for. Maybe things haven't turned out quite the way you expected - but they are still pretty darn good and when you look at things with gratitude you come to see that there is a lot of life worth celebrating and being happy about!