I feel very strongly that the concept of friendship has changed greatly in my lifetime.
My grandparents' generation, and to a slightly lesser extent, my parents' generation had basically made friends with a set of people during the early years of their married lives and retained these friendships into old age. They also had friends they had made at school, and put a great deal of time and effort into keeping up with family - both their own generation, the elder one, and our generation.
However, most of them lived their entire lives, or their entire life until retirement in the same place.
A great many people of our generation have not done the same. We chose to drop the relatives we only saw or wrote to "because they are family" and concentrated on the people we knew who shared our interests.
Naturally, some friends faded out of the picture, as the interest that had originally bound as together (young children, a place of work) changed.
My generation also experienced that those of us who married late, were automatically excluded from the lives of our contemporaries who had married and had children. No-one apparently wanted a single female, or a childless woman in their circle of newly married, nest-building happiness.
Are not nearly all friendships situational these days?
I have two school-friends left out of two school classes of 24 girls, two sisters who have known me since I was born, no friends from my college and university years ( death has been busy there, and others have moved to other countries or cities) and only one former colleague who has felt the desire to keep up with someone who had retired and left town.
I am not being self-pitying , just describing what seems to be the common experience here.