As the novel opens, Scrooge feels beset by Christmas visitors either wanting to invite him over on Christmas or asking him to donate to the poor. In response, Scrooge expresses his desire for isolation:
“I wish to be left alone,” said Scrooge. “Since you ask me what I wish, gentlemen, that is my answer. I don’t make merry myself at Christmas and I can’t afford to make idle people merry."
The effect of the above on the reader is to create a feeling that Scrooge is a cold, selfish, and hard-hearted person who is missing out on the best of what life has to offer. Mixed in, perhaps, is our disgust that he won't help the poor or enjoy Christmas is pity for his way of life.
When Scrooge revisits his youth with the Ghost of Christmas Past, we are able to witness what a lonely child he was during the Christmas holidays. In the passage below, he glows with happiness remembering a time when visitors came in costume to see him:
“Why, it’s Ali Baba!” Scrooge exclaimed in ecstasy. “It’s dear old honest Ali Baba! Yes, yes, I know! One Christmas time, when yonder solitary child was left here all alone, he did come, for the first time, just like that. Poor boy!" [ ... ]
To hear Scrooge expending all the earnestness of his nature on such subjects, in a most extraordinary voice between laughing and crying; and to see his heightened and excited face; would have been a surprise to his business friends in the city, indeed.
This passage raises our sorrow and sympathy for the lonely little boy Scrooge once was. We also see from the second paragraph that Scrooge changes into a different person, one who can feel emotion and remember joy, as he remembers what it feels like to have company. This shows that sociality has a beneficial effect.
In a third quote about his childhood Christmas experiences, we learn:
there he was, alone again, when all the other boys had gone home for the jolly holidays.
Once again, we feel sorrow for the neglected child. However, his sister comes to take him home, and we feel and enjoy his deep delight at being pulled into the world of human community.
All of the quotes show the negative and corrosive effects isolation has on a human soul. Being in community raises Scrooge's empathy and softens his heart.