The thing is, it’s not a case of go to a book club or volunteer at an event, and suddenly have best friends.
I think of it as concentric circles. In the biggest, widest circle you have all the people who are potential friends.
The next circle in are the people you have some interaction and familiarity with. You smile and say hi, but you don’t know their name, eg the woman who is often in the gym changing rooms at the same time or who does the same cooking class as you.
Circle three people are a bit closer. You probably know each others real names and you past time about the weather, the traffic, Christmas shopping etc.
Circle four, are the people you deliberately choose to spend time with. Crucially, you have both deliberately made the decision to spend time with each other, eg staying for a coffee after the class, run, walk, event etc. You talk about your family, work, hobbies, weekend plans with these people.
Then you have a circle of the people you can make commitments with. You plan to go out to a garden centre or out clubbing. You have moved beyond the context that first brought you together.
The penultimate circle are your intimate friends. You can trust them with your mistakes, shame, deepest hopes etc. You can call them in a crisis.
The final, centre, is you. The bit that must hold together strong without anyone else.
People generally move from the outside, in. Some conditions can speed this up, eg starting uni together or attending a Meet Up group.
As you move from outside, in, the level of intimacy and quality of interaction increases. But so does vulnerability and risk.
My final thought is that people have different expectations of how quick the movement inwards in should be, or even if they will move in at all.