Agree with Paul that you’ll find a lot of what has sparked your interest here in interaction design, and if you deal with tools that facilitate any kind of communication and social behaviors, social interaction design. Psychology is a vast and unruly discipline, and in itself inadequate to the construction of design methods or user research approaches. I’d combine it with sociology and anthropology (including ethnography) for insights into action. Action theories are incredibly useful IMHO to UX, IxD, SxD insofar as they provide a framework of understanding of both individual “user” actions as well as mediated social interactions. Individual actions are self-oriented (user is satisfied when internal conditions are met — we sometimes call those “needs,” “requirements,” etc but there are emotional and other factors as well, as you point out). Social interactions are socially-oriented and satisfied when conditions of successful communication or activity outcomes are met (e.g. mutual understanding, agreement, transactions, etc).
I also personally believe that media theory and theories of technology and society are incredibly helpful, in that we interact with technology not on its own terms but always situated within cultural and social schema. A great deal has been written on these, particularly since the 60s.
