Thank you for reading and taking the time to comment! These words struck me:
Snape was caring, devoted, and unfaithful.
because that's EXACTLY what I was aiming for! *hugs* I was really hoping to capture the contradictory nature of people--the realism of being happy and yet not overly cautious to preserve it, and even reckless with it. I was on the fence when it came to drawing out more motivation for Snape's infidelity--one the one hand it makes more sense and would probably be more satisfying to readers to see a concrete reason (Snape's a bad guy, Harry's a bad guy, the relationship is a little tired, etc) but on the other hand I couldn't get over the feeling that cheating can be one of those ordinary funny things that just happens in life. It'd be a one-time thing, an oops-wrong-turn and then you backtrack sort of situation. And sometimes, you don't suffer for your mistakes, because nobody knows but you. Here, Snape says he doesn't even see it as a mistake--he had a chance for something pleasant to him and took it--although I'm not sure yet if he really thinks that, or is just rationalizing.