Oxygen
A relationship needs new material coming in, from more than one source, and both of you have to keep supplying it.
You each have a life outside this and bring things back from it. There is something in the diary to look forward to. Neither of you can always guess what the other will do next.
Every conversation is logistics. Nothing is planned further out than the weekly shop. You have told each other everything, and one of you has started finding the interesting part of the week somewhere else.