The way I see it, Scrooge had a shock to the system (sort of). First he sees the door knocker change to be Marley's face. Then he sees Marley's ghost with all the chains. Then he get ghost of the past and goes to the past. He sees himself all alone in the school. Then we see him a little bit older and learn his father didn't want him at home and he had a sister that died in childbirth. We also see him at his first job and he falls in love and that falls apart as he gets more successful and wanting to make more money.
Next he gets the ghost of the present and we see Bob Cratchett and his family and Tiny Tim. Scrooge did not know the circumstances Cratchett was in and the situation with Tiny Tim. He also sees his Nephews house and how happy they are playing silly games and just being together.
Finally he gets the ghost of the future and learns of a man who died and nobody liked him at all. That life is better off without him. Also, he's found out that Tiny Tim has died. He finally finds out that it's his death and that really gave a shock to him.
All of these things combined really has show Scrooge the light. It's changed him. He didn't want to be that man who died alone. He didn't want Tiny Tim to have died. He changed so what he saw in the future would not come to pass.
He knew he could do something about the rest of his life and he did.