I think you're coming from a place of a young adult. Anybody with kids will see it quite differently - there's a lot more to it than mate selection :)
As for kids being happy - they just have near infinite energy to spare. As your responsibilities increase and your infinite energy begins to whither, it's a different dynamic.
Could you expand on your point? I'm a kindergarten teacher and I'm currently doing a developmental psychology course. While there is a lot more to it than mate selection that is very, very important. Erikson said the main task or crisis of the adolescent phase is identity versus identity confusion but developing one's own identity is a necessity for all facets of adult life, mate selection included. The hollowness of most adolescent experience; Go to school, learn what we tell you to, do as you're told, the general high school experience means that socialising is where most adolescent's mental energy goes. Since this is a zero sum game and a very large part of it is mate acquisition I don't see that there's that much more to it than mate acquisition.
As for kids being happy - they just have near infinite energy to spare. As your responsibilities increase and your infinite energy begins to whither, it's a different dynamic.