I think that one can find much in way of both theories helping to explain why we enter relationships. I don't think that each notion rejects the other. Both can work together with other ideas in helping to explain why we enter, strive to sustain, and endure our relationships. The need to belong, at its most basic core, is one of the motivating factors in a relationship. "Patterns of group behavior and close relationships" help to meet this need. It is something that Baumeister and Leary found essential in understanding why people do what they do. At the same time, I think that Bowlby's theory of attachment serving a fundamental need in who we are and what we seek in others is equally important. There is something to the idea that monotropy motivates the individual. The idea of being able to find an individual who will meet our needs and represent the focal point of our experience is vital to understanding why attachments form. If Bowlby is right, the attachment relationship which sustains the child's model of their own interaction with others is started at an infancy stage and is something that drives the individual in many emotional realms. Certainly, taking this notion along with the need to belong can go very far in explaining why individuals seek out and desire relationships on many levels.