The Ghost of Christmas Past is most effective on Scrooge because he reminds him of the person he used to be.
More than any of the other ghosts, the Ghost of Christmas Past makes a huge impression on Scrooge. Marley’s ghost makes Scrooge thoughtful, and helps prepare him by making him more receptive to the emotional onslaught of the three ghosts of time.
The very first thing the Ghost of Christmas Past does is shock Scrooge and get him off balance. He does this by making him fly and then taking him to his childhood school. Scrooge is not ready for this, and the effect is immediate and telling. Seeing his childhood makes him as giddy as the schoolboy he once was, and allows him to more easily accept the lessons that the ghost is trying to give him.
When Scrooge first wakes, he is preoccupied with the time. He sees the ghost and asks him who he is, and is reluctant to listen to him. The ghost forces him to go with him, and Scrooge’s attitude immediately changes.
They walked along the road; Scrooge recognising every gate, and post, and tree; until a little market-town appeared in the distance, with its bridge, its church, and winding river. …. All these boys were in great spirits …. (Stave 2)
When Scrooge sees himself as a little boy all alone at school, he feels pity. This pity is a monumental occasion, because up until this point he has not felt pity for anyone else. Once he feels sorry for the little boy he was, he remembers the little boy who sung carols.
“Nothing,” said Scrooge. “Nothing. There was a boy singing a Christmas Carol at my door last night. I should like to have given him something: that's all.” (Stave 2)
This sympathy, and the regret that comes with it, prepares Scrooge for the rest of the events the ghost shows him. As he joyously experiences Fezziwig’s Christmas party and feels the sting of his fiancé Belle dumping him, he starts to feel feelings that he had locked away for years. Scrooge is one step closer to being human, and ready for the lessons the other ghosts have for him.