I was counseled two days ago with this analogy:

A long term relationship is like a marathon, the longer you continue it, the more difficult it can become.

I think some people have a possibly incorrect expectation about relationships – they perceive the longer you are in a relationship, the more “comfortable” and “easy” it should become.

But the only people having human relationships these days are humans, and we are a fairly sophisticated, desirous, and particular species.

You may have more success in your relationships by not expecting they will become easier, but rather expecting they will remain relationships between two different, not-identical, and independent-minded humans.