This paper concerns the credibility problem for commitments. Commitments play an important role in cooperative human interactions and can dramatically improve the performance of joint actions by stabilizing expectations, reducing the uncertainty of the interaction, providing reasons to cooperate or improving action coordination. However, commitments can only serve these functions if they are credible in the first place. What is it then that insures the credibility of commitments? To answer this question, we need to provide an account of what motivates us to abide by our commitments. We first discuss two conceptions of the nature of the commitments present in joint action and of the norms that govern them. We contend that while normative considerations may have some motivational force, there are reasons to doubt that they, by themselves, could provide a sufficient motivational basis to fully explain why agents abide by their commitments and thus why their commitments are credible. In the next two sections, we discuss two proposals regarding further sources of motivation, reputation management and social emotions. We argue that while reputation management and social emotions certainly play a role in motivating us to act as committed, there are both theoretical and empirical reasons to think that neither captures the most basic motivational force at work in sustaining commitments. We propose instead that the need to belong, i.e., the need to affiliate with others and form long-lasting bonds with them, is what primarily motivates us to interact and engage with those around us and act so as to preserve and reinforce the bonds we have forged with them. We argue that the need to belong is a more basic proximate motivation for conforming to commitments, in the sense both that affiliative behaviors are evidenced much earlier in human development than either reputation management or social emotions and that the need to belong is at least part of an explanation of why we care for our reputation and why we care about others’ assessments of our behavior.