Commonality or lack of same? Change is the only constant in humanity that I've seen. In H.S. your base of experience is fairly equal. Your world is rather homogenous.
Post H.S. begins the separation process of one aspect of life and begins the next homogenous environment of college, military etc. You also begin to develop a more complex version of "you". You begin to shape your value systems and your world view on your own experience rather than on the second hand opinions and experiences of others, such as your parents and teachers. Your base of experience is no longer the same. Your world is very heterogenous now. My friends were in a bunch of different colleges, I was either at Ft. Bragg or in countries that geography major would be hard pressed to find on a map... you have less and less in common. Silence due to absence is easier to accept than tension due to presence. In other words memories of what things were like are sometimes better than the fact that you can't agree on anything anymore.
Post college (mid to late 20's), you become wrapped up in establishing yourself, changing the world or at least blowing up 35% of the planets surface. Putting down roots perhaps, or the opposite, becoming comfortable as a gypsy. Either way you spend more time developing that sense of your self, your world, and your place in it. Some are also a bit more competitive. I remember hearing that my 10 year reunion was more a case of who's got what, who's doing what, who's doing the best...
Now I'm married more settled and I find myself talking to old friends from HS, college, and the military more often. Most of us are married and in fairly stable careers, some have children, some are still practicing

. Marriage and family has brought about a new type of commonality. So it begins again. It's all cyclical. Seeing that some of you are from Shepherdstown reminds me of a friend I had at one point who went to Shepherd. and I wonder what happened to her...
Anyway, that's my doctoral thesis on the topic...
andy