I apologize for the lateness of my reply. Tuesday is my long day at work, and I had something to post in the evening, so I simply hadn't gotten to it until tonight.
I loved the accumulating moments that led to Harry's observation, and his combination of innocence and obsession. Severus describing the appeal of being bound was beautiful, and one of those moments of strange eloquence that is so much in his character. The next scene between them in the office is brilliant, from Harry's "Kill me now" to Snape's underhanded challenge.
I can't possibly quote all the parts I like, but this was stunning:
But Snape's securely tied body was no match for the beauty of his face. The more ropes Harry bound him with, the more relaxed he became. The pinched expression Harry had become so used to was gone; Snape's lips were slightly parted, his eyelids gently closed.
I love the idea of Severus becoming beautiful as his tension fades from him.
Harry's need and how he pushes it into action is equally wonderful.
Thank you, mystery writer! You did a wonderful job with this prompt.