Friendship is like money, easier made than kept.

Starting a friendship takes an evening. Keeping one takes years of small unprompted effort that nobody applauds and nothing forces. That asymmetry is why people in their forties can have met a thousand people and be close to four, and it is not usually because anything went wrong. Things simply require maintenance, and maintenance is invisible until it stops.

Pick the two or three you would be sorry to lose and do the unglamorous thing: the message with no purpose, the call that is not about arranging anything. Frequency matters more than depth here. What ends friendships is rarely a rupture, and almost always a slow default.