When working with a couple where one partner has had an affair the question of why it happened always comes up. The conventional wisdom is to look to the deficiencies of ‘the relationship’ – what did not happen in the relationship that should have. This can cause resentment from the betrayed partner who says “hey I did nothing wrong or anything different from before! Are you saying it’s my fault?” As expected it depends on the situation as to the causal question. One observation that I have is that it is important for the person who strayed from the marriage to look within themselves for the answer as to why. Perhaps there has been growth and change and poor communication skills or discomfort with intimacy has led him/her to look elsewhere for answers. Maybe there is a legacy of values passed on from the family of origin (ie. is divorce ok) Maybe there is a substance abuse problem that has contributed to impulsivity and lack of judgment. I almost always recommend individual counseling for the straying partner to develop more understanding and insight into themselves and what contributed to their willingness to entertain the option of going outside the relationship.