I think it's the matter of factness of the narrative voice that makes this short story so poignant to me. I can see the beginnings of the man we "meet" in PoA here. While Sirius basically struggled to hold unto himself in one horrific place for 12 years, Remus had a journey to make to become Professor Lupin. Your spare story shows just enough to make it feel real and honest. And nothing more so than their reunion.