This effect and defect certainly comes from ones emotion and mood. One could be experiencing more downs than ups in the relationship which in turn can be built up over time. After a period of built up from dissapointment one of the people in the relationship could call it quits because their needs are not met. This can all stem back to the model of self actualization. Within the self actualization model one needs many aspects to feel complete and whole. The need to feel wanted and safe from your partner are major things when it comes to ones self esteem.
We cannot really think of two people breaking up to be such a bad thing. Everyone needs to look at it as a positive thing because the chemistry had been lost. The honeymoon effect should be present for everyone at the start of their relations and should for the most be able to be rediscovered if lost. In the end if really boils down to one's emotions and what they are willing to put put. The motivation needs to be there with both partners.