Jamieson, Fausset and Brown on Galatians 4:18

18. good to be zealously affected--rather, to correspond to "zealously court" in Ga 4:18, "to be zealously courted." I do not find fault with them for zealously courting you, nor with you for being zealously courted: provided it be "in a good cause" (translate so), "it is a good thing" (1Co 9:20-23). My reason for saying the "not well" (Ga 4:17; the Greek is the same as that for "good," and "in a good cause," in Ga 4:28), is that their zealous courting of you is not in a good cause. The older interpreters, however, support English Version (compare Ga 1:14).

always--Translate and arrange the words thus, "At all times, and not only when I am present with you." I do not desire that I exclusively should have the privilege of zealously courting you. Others may do so in my absence with my full approval, if only it be in a good cause, and if Christ be faithfully preached (Php 1:15-18).