What led to the transformation of Scrooge from a miser to a generous man?

Scrooge's change is the result of his being visited by the ghost of his old partner, Jacob Marley, as well as three spirits: the ghosts of Christmas past, present, and yet to come.  Jacob's ghost sets the stage for the others, showing Scrooge that the purpose of life, really, is to help others.  He allows Scrooge to see other spirits who mourn their inability to assist the living; these spirits want nothing more than to...

Scrooge's change is the result of his being visited by the ghost of his old partner, Jacob Marley, as well as three spirits: the ghosts of Christmas past, present, and yet to come.  Jacob's ghost sets the stage for the others, showing Scrooge that the purpose of life, really, is to help others.  He allows Scrooge to see other spirits who mourn their inability to assist the living; these spirits want nothing more than to help the poverty-stricken, but now they are no longer able.  Instead, they have only their chains, their pain, and their regret.

Then, the ghost of Christmas past shows Scrooge scenes from his youth, scenes that help him to understand how he began to change from a hopeful child into an embittered adult.  The ghost also shows him how pleasant it was to have a generous boss.  The ghost of Christmas present allows him to see Christmas at his employee, Bob Cratchit's, house, how meager the feast is and how grateful the family is for it.  He also sees the dinner at his nephew's house, an invitation he'd refused, noting that his nephew seems to have a genuine concern for him.  Finally, the ghost of Christmas yet to come shows him the death of Tiny Tim, Bob's sick child, as well as his own passing, mourned by no one, indeed, even robbed by his former servant.

All of these visitations help Scrooge to see that he has made decisions based on money, and those decisions have rendered him alone.  He seemed to fear being abandoned, as he was as a child, and so he clung to something that could never leave him: his gold.  Once he realized that sharing his money would result in happiness, not just for those whose loans he's forgiven but for himself as well, he determines to give generously.