The rubric that governs the ethos of this [the Christian/Galatian] community is a formula of reciprocity as creative as it is paradoxical. The Galatian freedom will not become an opportunity for “the flesh” inasmuch as they are “slaves to one another through love” (διὰ τῆς ἀγάπης δουλεύετε ἀλλήλοις [5:13]). This is a remarkable expression since it adjusts an inherently hierarchical relationship (slavery) not by canceling it, in the name of “equality,” but by making it reciprocal, a hierarchy that turns both ways. The simple but powerful word ἀλλήλοις turns a one-way relationship of power and superiority into a mutual relationship of reciprocal deference, where each seeks to promote the interests of the other.
Barclay in Galatians in Christian Theology