How the thinning happens

It's worth laying the whole mechanism out once, because the habit depends on each step looking like nothing.

A message arrives. You like the friend and you want to answer well, so you attach a standard to the reply. The standard makes the reply a task that needs conditions, and the conditions aren't here, so the message stays open. That's the first day. On the fourth day the stretch has grown enough that the reply now needs a surcharge, a "sorry, mad week" at the top, and the surcharge makes the reply slightly bigger and slightly less pleasant, so it stays open longer. By the second week the turn has appeared, and the silence has a reason. By the fourth week it's been too long, and the reply has become a catch-up, a big message that has to cover a month, and catch-ups need a desk and an hour and a good mood, so it waits for those. The friend, meanwhile, has done chapter 12, and sends the next message a little later and a little shorter. You answer that one faster, because the previous one is still open and this one lets you skip it, and for a while things seem fine. Then the next one doesn't come.

Nothing in that sequence is an event. Each step is a day on which a reply that could have been sent wasn't, for a reason that was true that day. The reason changes as the stretch grows, from the standard to the surcharge to the turn to the catch-up, and each reason is bigger than the last, so the reply gets harder as the reasons for sending it grow. That's the shape of it. The longer you leave a message the more welcome a reply would be, by chapter 6, and the less able you are to send one, by this chapter.

A tap left dripping doesn't flood anything, and after a year the bill has come and nobody remembers turning it on.

I'd add one more piece, which is about how the thinning stays invisible. You don't see it because there's no day on which you had the friend and a next day on which you didn't. What you see, years later, is a list of names you're fond of and don't speak to, and a story about how people drift apart, which is true and which leaves out the part where the drifting was made of days like today. The friends you've kept aren't the ones you liked most. They're the ones where, for whatever reason, the replies kept coming.

The whole mechanism turns on the first step, the standard. Take that off and the message closes on day one, before the surcharge, before the turn, before the catch-up, and none of the rest happens. The method in chapters 19 to 21 is mostly about that first step, because it's the only one that's cheap.