Interesting thread!
Most friendships ime congregate around situations - schools, work, clubs etc. All of which come and go as life rolls by. And most of those friends don't 'stick'. Not a source of worry for me, it is what it is.
Other things that kill friendships over time ime are situational changes such as friends who suddenly become doting grandparents and their life focus shifts and we no longer 'gel', some just become older, duller and lose their vitality, and some become over-clingy and just want too much of your time. Various people have been de-friended over the years because of these reasons. Again, this doesn't bother me.
It helps that I do not tend to be a backward-looking person. I don't bother with reunions, I don't yearn to reconnect in any way, shape or form with ex's - in fact I cannot understand what people get from such outreaches. Maybe just a longing for yesterday and perceived halcyon days.
I'm a very sociable person and like to have friends to engage with but I do accept that, apart from a few, that these will almost certainly be transient relationships.