Re: If you have to mention just one reason....
Lack of communication....which leads to either or both parties presuming what the other is implying...usually they presume the negative.
And from then on it spirals downwards so fast before you know it you are in a such a rut that it becomes impossible to be together. Even though it is painful at the time, you must communicate...If you have to then even fight about it ...it can lead to some sort of solution rather than keep quite and simmer in bitterness and incorrect presumptions.