But they don't, as you get older you kind of find the things you fundamentally like and dislike in life and learn to accept them, it's perhaps one of the few perks of getting older, knowing yourself, knowing what makes life enjoyable and what doesn't.
People might be happy with their life, but this doesn't mean that they wouldn't be happier with an alternative life. After they try the alternative, they could even think that what they had thought as happiness, turned out, was suffering.