A relationship can have no clear ending because neither person wants to be the author of it.

Some relationships have no ending because ending one requires someone to author it, and neither person wants that role. So it thins out instead, through unanswered messages and delayed replies, until it has effectively finished without anyone having decided. The ambiguity protects both people from being the one who did it.

That drift is comfortable and it leaves a particular kind of unfinished feeling behind. If you want something to end, saying so is kinder than fading, even though it costs more. And if you want it to continue, the same principle applies: name what is happening, since the drift will otherwise complete itself without either of you choosing it.