So utterly real! This is the Severus who lurks behind the Snape we see in canon: a working-class boy embittered and defensive, and so friendless he's talked himself into believing he prefers to be alone. Until he's confronted with another stunningly realised character: Harry with his guilt and his Saving People Thing and his sheer stubbornness all drawn in perfect, pin-sharp detail.
This is a masterpiece, a true tour-de-force, a triumph of characterisation and observation and emotional intensity.