"'What reason have you to be merry?'" (622) Though this comment is meant more out of spite than anything else, it reveals that Scrooge isn't happy, so why should anyone else be?
Everything a person does will influence the yet to come
"At one of these [desks] a lonely boy was reading by a feeble fire; and Scrooge sat down upon a from, and wept to see his poor forgotten self as he had used to be." (635) This is a moment from his past, one of many that show why Scrooge is so bitter at Christmas. Who could forget being left behind every year at Christmas because his father didn't want him to come home.
"A room, not very large or handsome, but full of comfort. ... The joy, the gratitude, the ecstasy!." (639) The woman is who Scrooge would have been married to had he realized how precious their love was. Instead, he was single, alone, and with no family to bring him joy. Because of poor decisions from the past, his present was dark and cold without love.
"'Are these the shadows of the things that Will be, or are they the shadows of the things that May be only.'" (663) Scrooge is shown his own grave, after a length of seeing others happy that this man has passed. He does not believe that this could possibly be himself, as everything he has been shown on this night has led to hope, not despair and loneliness.
"He became as good a friend, as good a master, as good a man as the good old City knew, or any other good old city town, or borough in the good old world." (669) He changed because of what he had seen on that night, and the grave of that man would not be his own.