One of the main reasons was the constant comparing of the partner with perfect images. About it tell results of research of scientists from the University of Texas.
They conducted an experiment involving 260 volunteers with experience in long-term relationships that lasted 7 years and more. Subjects were asked to describe the 27 characteristics of an ideal partner for a relationship. Next, these parameters were required to rate their loved one.
Scientists have noticed that the happiest people were those who lived near his own ideal, and not tried to remake man to suit your requirements. Apparently, for a meaningful and lasting relationship you need to love a man the way he is, and not try to change it.