In A Christmas Carol, why does Scrooge have any doubts that he will change the future he saw?
He is most uncertain because of the reaction, or perhaps the lack of reaction, he gets from the Ghost of Christmas Future.
The spirit has shown Scrooge many disturbing things, and Scrooge is petrified that that all of this will come true. Remember, though, that Marley told him at the very beginning of the story that he was going to be visited by three spirits so that he might change his ways. So, Scrooge is hoping that Marley is correct and that if he changes he might save himself and others from the terrible fate he has been shown.
"Men’s courses will foreshadow certain ends, to which, if persevered in, they must lead,” said Scrooge. “But if the courses be departed from, the ends will change. Say it is thus with what you show me" (Stave Four)!
To this, the spirit says nothing and then simply points to Scrooge's gravestone.This leads Scrooge to worry even further because the spirit is not confirming that if Scrooge changes his behavior that it will change the future. His silence causes Scrooge the most doubt.