Growing apart. Which results in @stanleybmanly‘s incompatibility.
All relationships end. Some by death, the rest because something changes or is not growing in the way both people want. The biggest obstacle to finding a relationship that last more than a few months is communication.
Look through the list above, most of those issues should be resolved or discussed before, as the OP says, ”...after being in relationship for say few years or so…”
@stanleybmanly “Incompatibility” is listed because it’s a catch-all. Any other reason might require all kinds of proof. There was a time when getting divorced was a lot harder than it is now.
I have a feeling that most married couples fall out of “love,” but “successful” marriages stay together anyway. My 1st marriage was not “successful,” and I’m still glad I got out. I think.