Why does the ghosts show Scrooge the unkindness of his father?

The Ghost of Christmas Past wants to remind Ebenezer (and the readers) why he became embittered against the holiday, and against humanity in general. Part of the reason was that Ebenezer's father would not allow his son to be with his family, even at Christmas -- leaving Ebenezer to "celebrate" the holiday alone, and in terrible pain. When reminded of this fact, the older Ebenezer "sobbed."