Because it is the typical path of being a human being. If adults were going around having innocent fun after puberty there wouldn't be human beings now.
> (a pathetic cocktail of joy and hurt) centered on love?
It is different and personal of course. But I wouldn't want to live a life that didn't have that. It would boring and un-exciting.
> now-adult human forgets all about that childhood joie de vivre (a cheerful enjoyment of life; an exultation of spirit)
Not all is lost ;-) you can still find that when you have your own children! When I play with my kid, I am a kid again. We make paper airplanes, play with dolls, play tag and so on.
And I belive you get one more chance, when your children have their children!
I think depending on the perspective life can be very exciting (but even writing that in words sounds trite and boring).
My confidence about love's failures comes from not only my own feelings, but also observing others.
> You in another decade may feel differently, and any other person will of course have different value judgements and desires.
I'm yet to come across someone who have demonstrated that love works ... and by "work" I mean it being an one-hundred percent fun way to go about life.
Love is something that happens during life, it's part of it, inside it.
Questioning love = paying attention to your own feelings, and questioning their validity. Why do children seem to be having innocent fun until puberty sets in after which the now-adult human forgets all about that childhood joie de vivre (a cheerful enjoyment of life; an exultation of spirit) and sets about to religiously believing in a second-rate life (a pathetic cocktail of joy and hurt) centered on love?