There are numerous couple issues, i.e. negotiating 'power and control', sex, money, commitment, all with their unique signature details for each couple. However, in my experience, aside from dealing with above dynamics, 'tolerating intimacy' is the key to resolving the issues that couples encounter.
Teaching that which sounds like a contradiction is a focus in my work with individuals and couples. The concept comes from how you observed both your parents relate or not relate intimacy to each other, and how each parent conveyed intimacy, or not, to you.
This triangle of 'intimacy' or emotional climate, becomes one's unconscious template of intimacy that each person seeks to replicate. It's an in-depth analytic explanation of 'love'.
The psychoanalytic writer and object relationship theorist, Ronald Fairbairn, felt love and the relationship (with one's parents) was the same thing. It's the only concept in my own analytic training that finally explained true sado-masochistic relationships. So while whips and chains are the extreme example, many couples engage in hurtful relationships that fall somewhere on this big conceptual curve.
Many years ago I summarized this concept in a single sentence: 'Love is that which the relationship was.' So if 'love' hurt, the child/adult would continue to push for this type of 'love' in the present relationship.
It's hard getting people to change and this is what I explain as the focus of my practice. I help patients to let go of their old 'love objects' and embrace their present partner/new love object. Old defenses are greedy carnivorous beings, but with motivation they'll let go if you offer them a new trade in the present - your partner.
See also: Body—Mind Strategies for Dealing With Anxiety and Depression