At first, Ebenezer Scrooge thought that the reason why Marley's ghost had appeared was indigestion. The ghost spoke to Ebenezer and so Scrooge thought that another reason for this ghost to be there was the friendship these two men once had.
Ebenezer Scrooge kept trying to rationalize what had happened, and the more he tried to put his thoughts to rest the more he kept thinking about it.
Scrooge was trying really hard to give a reasonable and believable explanation to Marley's ghost appearance.
