In childhood and adolescence we easily make new acquaintances, we tend to call every other person our friend, and every fifth friend has a serious chance of becoming a best friend. The "collection of friends" tends to expand until about the age of 25, after which the decline begins. As we approach our thirties, we make fewer and fewer new friends or even close buddies, and the connections we already have go through a serious revision.
Regardless of how many friends you manage and want to make in adulthood, the "You're my best friend" bonding period, as it was in adolescence or young adulthood, is now over. Now you make "sort of friends" - buddies, acquaintances, close people - and often you yourself do not understand what epithet to give to this or that person.
When we are younger, we are more inclined to go to extremes in defining for ourselves what it means to be friends. Hence, for example, the making up of various friendship oaths - friends are seen as people who are often closer to us than even siblings. And anything even slightly less than complete loyalty on their part implies a break in the relationship. All or nothing!
So there are only two things left to say in closing. First, thank those people who have remained your friends in spite of the years. Secondly, right now write to that acquaintance whom you have long wanted to offer to meet in the evening after work.