Some years ago, I went for salsa classes but had to drop out. One of the young men there, just a teenager perhaps, was the spitting image of one of my childhood abusers. We would dance in a circle, and change partners every few bars. I tried not to look at his face when it was his turn to dance with me. But then came the step where the man stands behind the woman, crossing her arms over as if pinned, and holds her from behind. I had to excuse myself "to go to the lady's room", and walked calmly down to my car, where I broke down and wept for ten minutes.
I never went back to salsa classes. Snape's expression as he looks at Harry in this picture reminds me of that moment. Memories of evil triggered by an innocent face.