The Friendship That Fades Through Nobody's Fault
Most friendships do not end in a rupture, they end in a series of individually reasonable postponements.
Ask people about lost friendships and they will describe an argument, because arguments are memorable. But the great majority of friendships that disappear from an adult life do so without any incident at all. A message goes unanswered for a week because the week was bad. The reply, when it comes, is short. The next round takes a month. Each individual delay is entirely reasonable, and there is no point at which either person does anything wrong. Then a year has gone and you are people who used to be close.
The reason this is worth naming is that a fade is almost always experienced privately as a rejection. You notice you are not being contacted, and you draw a conclusion about your worth to somebody, on evidence that is really just evidence about their diary. Meanwhile they are doing the same in reverse. It is possible for two people to independently conclude that the other has moved on, when neither has, and for both to be entirely sincere about it.
The remedy is unglamorous and quite effective: assume nothing has happened. If it has been long enough that reaching out feels strange, that is exactly the moment reaching out is worth the most. Do not open with a defence of your silence, which invites them to account for theirs and turns a friendly message into a small tribunal. Just resume. Friendships tolerate enormous gaps if nobody insists on making the gap the topic.