All right, I can't claim to have actually grown up, but as much as I have experienced here is what I have figured out:
--As a kid you are allowed to be happy, at least about certain things part of the time, a main one of which is what you will do or be when you grow up.
--The whole time you get older, as Dewey on "Malcolm in the Middle" famously said, "Every year they expect more of you and love you less." You keep hoping things are either not as bad as they appear, will get better, or both.
--Finally your life is reduced to nothing but worrying about a bunch of insurmountable problems. God forbid if you try to concentrate on something you enjoyed when you were younger and not on these problems, something terrible will happen, but then if you do nothing but obsess on these problems, something terrible is likely to happen anyway.
--You then realize that everything was impossible all along and there is no way you can prevent it's being terrible, you can only ignore or postpone it. What we call adolescent angst is the tip of the iceberg here. At that age they don't know everything, they have just learned enough things a young child doesn't know to start being really miserable.
--As J. M Barrie observed, "Nothing that happens after you are twelve years old matters very much." End of story.