Guys why is there such thing as a Midlife crisis and why just because your in your 40's or so, why is that bad?

lol just wondering why is there is such a thing as a mid-life crisis other then menopause and to fight aging, i never got why so many peeps possibly middle-aged always say mid-life crisis i don't get, cause peeps in there 60's and 70's are always laughing and happy and i don't get what the crisis is in middle age that gets people down. Thanks, please no jerks neither and just be a helper.