Why does the Ghost of Christmas Past show Scrooge the boarding school where he was left alone?

Ebenezer Scrooge lives a solitary existence. Rich in material wealth, he has eschewed the company of man seemingly destined to die as he has lived -- alone. Charles Dickens, in his story A Christmas Carol, portrays Scrooge as miserly and perpetually bitter, treating all about him, especially his loyal employee, Bob Cratchit, dismissively and contemptuously. Having depicted his "protagonist," early in his narrative, in such a negative, unlikable way, Dickens then proceeds to follow Scrooge as the elderly businessman arrives at his home and proceeds to his evening routine, all alone. In Stave II, as warned by the ghost of Scrooge's deceased business partner, Jacob Marley, Scrooge is visited that night by a series of spirits, the first being the Ghost of Christmas Past. It is, of course, during the old man's encounter with the first of the three ghosts that he is transported back in time to a reminder of the solitary existence he lead even as a child. As the ghost and Scrooge visit the scenes of the latter's early life, Scrooge is elated to recognize individuals from his past. As described by Dickens, the scene is as follows:

The jocund travellers came on; and as they came, Scrooge knew and named them every one. Why was he rejoiced beyond all bounds to see them! Why did his cold eye glisten, and his heart leap up as they went past! Why was he filled with gladness when he heard them give each other Merry Christmas, as they parted at cross-roads and bye-ways, for their several homes! What was merry Christmas to Scrooge? Out upon merry Christmas! What good had it ever done to him?

“The school is not quite deserted,” said the Ghost. “A solitary child, neglected by his friends, is left there still.”

Scrooge said he knew it. And he sobbed.

This poignant scene in A Christmas Carol serves to enlighten the protagonist and the reader as to the origins of Scrooge's current demeanor. This is an important passage as it reveals Scrooge's inner need for the human companionship he has disdained in adulthood. He is overjoyed by the sight of these people from his long-gone childhood, only to be reminded by the vision of himself as a lonely child. For him, the joy in those around him never penetrated into his soul, and he would grow into a man seemingly content to maintain invisible walls between himself and the rest of humanity. It is this scene, as well as those that follow during the course of the night, that helps Scrooge to recognize the error of his ways and to begin a new chapter in his life, one that embraces those he previously spurned. The Ghost of Christmas Past has shown Scrooge his loneliness as a child in order to display for the old miser the length of the road down which he has traveled to reach his current state of being.