But sometimes the problems are not just internal.  They can be external.  Both of you have personal stresses that do not find their source in the relationship. 

Like I said, defensiveness happens in relationships.  But reacting from a place of defensiveness is harmful to the relationship.

Gottman discovered that an important predictor of relationship stability is whether couples make “cognitive room” for their relationship and for the world of their partner. This involves developing a map of the world of your partner, a cognitive map of the relationship and its history.

Gridlock happens because you are dealing with a perpetual problem.  These problems, which make up over 70 percent of relationship conflicts, are perpetual because they are based on differences in things like personality, life experiences, family-of-origin dynamics, temperament, or deeply held values.