Lack of mutual trust and understanding.
Both the partners should listen to each other calmly and try to identify each others problems and grievances.
Over possessiveness and over expectation can be other important reason from either side.
Each partner should be given their own space as well. One does not have to keep on telling, ' I love you', every time, but some kind of act can be done, so that a feeling of reassurance is felt, that the sincerity is there in relationship.
It takes a lot of time and effort to build relationships, so each partner must value it.