Physical and emotional abuse
If you are going through emotional and physcial abuse in marriage, this is one of the major causes of unhappy marriage. Abuse, at any level, is not acceptable and to have an abusive partner will only kill your self-esteem and ultimately, the relationship.
Poor communication
Lack of communication in the relationship leads many misunderstandings. When partners refuse to communicate or share their feelings, it is surely going to lead to a downfall.