I outline a novel theory of deontological constraints, based in the idea that these are the constraints we must abide by to participate well in interpersonal relationships. I understand deontological constraints as moral principles that forbid actions even when those actions cause no independent harm, or are even beneficial. Paradigm cases are the constraints against killing, promise-breaking, lying, stealing, and sexual assault. But why are these actions subject to constraints, rather than some other set? I suggest that we can explain the contours of constraints by looking at the nature of relationships. More specifically, I propose that relationships are best understood as activities of shared agency, and that if we unpack the norms one must obey to participate well in shared agency, we will find that these norms align neatly with the intuitive content of constraints. The upshot is that our reason to abide by constraints might be explained in terms of the value of interpersonal relationships.