To my mind it makes perfect sense that relationship is the key. What else could it be? Some specific skills? Then you need a book and a teacher, and I suppose that some therapies really are of that type (DBT?). But this skill-building approach cannot solve interpersonal issues. Interpersonal issues were created in the relationship and they can be overcome only in the relationship. The relationship is the only sensible context where such issues can be explored and healed. The relationship generates new experiences or novel interpretations of known experiences and that's why it is pivotal for healing.