At the time when Dickens wrote A Christmas Carol, the prevailing spirit in England was one of fierce individualism. People generally believed that you should look after your own interests and let others get on with living their own lives. In one respect, this is an admirable way of thinking, but it also has a downside in that it can so easily pass into a general indifference towards the poorest members of society.
That's precisely what happens in the case of Scrooge. His lack of concern for the poor is on display for all to see when he sends the two charity collectors packing after they try to hit him for a contribution. Scrooge doesn't believe in helping the poor; he'll attend to his own business, thank you very much.
Such a heartless attitude leaves Scrooge a lonely, isolated man. He has no friends, no social life, and no one to care for. He has money and lots of it, but that's no substitute for human connections. In portraying Scrooge this way, Dickens hoped that his readers, many of whom will have harbored similar attitudes to Scrooge, will realize that such rampant individualism and contempt for the poor can leave one feeling isolated.
Above all, Dickens wants his readers to recognize that all of society has an obligation to others, not just to ourselves. If we can change our attitude towards our fellow man, as Scrooge so spectacularly does after the visit of the ghosts on Christmas Eve, then we will develop greater empathy and emerge from out self-imposed isolation to share in the joys of mutual respect, care, and concern.