Imagine how awful it must be for Severus Snape to have to teach Harry Potter when Harry has his mother’e eyes (Snape was in love with Lily) and looks like his dad (Snape hated James Potter).
You’re stuck looking at the beautiful eyes of your (long-dead) beloved which reside in the face of the bloke you hated most at school.
Also, how does Harry not remember Snape cradling Lily’s body after Voldemort’s attack on the Potters? He was in the same room, and you can’t really forget someone like Snape! Surely that adds to Harry’s traumatic memories?