It may sound a bit hackneyed.....but there is a term that is often associated (at least, in my mind) with Hollywood celebrities....."Irreconcilable Differences".....used. at the time of their divorces.....
I used to laugh at that...as just a way to gloss over the "real" truth of their issues..to the public....
However, I, now, think there is a lot of merit to that in many marriages....
square peg and round hole situations....now compatable and not meant to be.....
Another thing that I have heard couples therapists say is...that....by the time that many couples come to therapy, together...especially after several years of conflict.....that one of the partners usually has one foot out of the marriage (emotionally).....and, that person comes to the therapy with the "secret" agenda of becoming validated for getting out of the marriage.....(this is usually never said, out loud).......
I'm just saying......