Here are some reasons:
The anger is not voiced. One partner or the other harbors resentment but does not bring it to the other’s attention.
The anger is not recognized. Not only do you keep it from your partner, but you keep it from yourself, either by denial or minimizing (“I’m not angry, I’m just annoyed”).
The issue behind the anger is not resolved. Unresolved anger does not simply evaporate; it remains in the relationship and saps positive energy that could be directed toward romance.
Sometimes anger is intentionally used to keep intimacy at a distance. A signal that this is occurring is when things are going along well in the relationship and suddenly one partner notices something annoying and picks a fight. Although the tension caused by the fear of intimacy is played out through anger, it is rarely recognized as such on a conscious level. Instead it often results in sexual indifference.