why do we crave to be alone so much more when we get older?
probably because we have first hand experience of the stress-inducing properties of socialization, leading us to the natural understanding that being part of a crowd is not something necessarily beneficial. we make our circles smaller, become sedentary, and fall into behavior patterns that suit our particular experience.
in short, it's because we spend so much time being forced to do things with other people (work, kids, friends, etc.) that, when we get the chance to actually do something, we choose to be alone and enjoy the peace.