And as I've tried to point out repeatedly (for example, here), in a very real sense there's no such thing as "private" sin. My lust, your pride, my laziness, your neglect of prayer, etc. affect each of us. When one member of the Body falls short, that negatively affects all of the Body, and when one of us has a spiritual victory in their life, it positively affects us as well. It's just like in marriage: My sin or spiritual victories affect my wife's spiritual condition and vice-versa.
I've thought this through some, and my interpretation of it (which I think is perfectly in accord with MacArthur's) is that when you sin, it's not just an issue of you breaking a law like going over the speed limit. As C.S. Lewis put it, when we sin, no matter whom else we hurt, there's One whose love was wounded in every transgression. We didn't just break a law: We broke our Father's heart, no matter how we've disobeyed him.