Qitzur Shulchan Arukh – 65:10
Even in the “benefit of pleasure” [ie: the satisfaction one feels in doing a favor for someone], which has no monetary value, it is forbidden for the lender to benefit from the borrower. For example, if lender is a craftsman, and this borrower does not usually, at other times, give give him work, only now because of the loan, he wants to give him work, it is forbidden.
I think this pretty much closes the case on my theories about the prohibition of interest. This halakhah explicitly takes it out of the domain of money, removing connotations of the loss of money being theft-like or otherwise immoral. And, second, clearly places it in the context of avoiding tit-for-tat exchanges of favor. The notion that this all revolves around the verse’s description of the other Jew as “achikha“, your brother, and therefore lending money should be a simple fraternal reflex, is at this point compelling. (To my mind. Feel free to disagree in the comments!)