Their partner may also have changed, for the better also. But combined, and without any fault, their tunes may sound unbearably dystonic. Consider what happens when we force ourselves to stay in a situation where there is persistent disharmony. I'm not saying the relationship is over at the first flat note. I am suggesting that many problems in relationships are the result of not finding the true function of a given relationship.
Relationships can be opportunities to work through any number of patterns.
For example, one could be drawn to someone that has a complementary sibling issue that is an obstacle to both. But potential strides are lost because Prince Charming has to find just the right foot. Some see this attractive force as the spirit of the relationship. I suspect many of us are familiar with the darker aspects of this spirit. Sometimes I think we have to be prepared to go through hell to pay homage to it.