1)During Auror training Harry realizes that his glasses become a hindrance. He seeks Severus help to regain complete eyesight. Upon Severus being the first person he "sees" Harry becomes obsessed with Severus face.
2)Severus and Harry exist within a universe where Lily, James, Remus, Sirius, Voldemort and the Dursley's do not exist. Both are the same age and in the same house(Slytherin or Gryffindor) at Hogwarts.