You need shared contexts and shared interests. The reason why it’s difficult to make friends as an adult is because where there's shared context (seeing the same people over many months daily i.e. workplaces), there's a lack of shared interests. Where there's shared interest (hobby groups, bookclubs, etc), there's a lack of shared context. A place
Danny Meyer the restaurateur has called ABCD: “always be collecting dots, so that you can always be connecting dots.” Dots or breadcrumbs, if your whole life is oriented around that collection, which mine is, then you're always preparing.