ships that pass in the night

The other day I re-connected with a friend from my university days - now this was my best friend in 1976.  As I thought about this, and the fact that we had parted ways, it occured to me that it would be iinteresting to chart all the friends and acquantances whose lives have crossed mine. In fact, best friends could be at the top of the chart , and then one wonders how a best friend becomes an ex-friend? Am I just fickle or is it usual for people to befriend and then move on like ships that pass in the night?