Answer:
There are some interpretations as to what is the theme in Hamlet. As I chose to see it, I believe that the theme is that people do not know how to deal with being mortal. Hamlet is mournful of his father and does not want to celebrate the new wedding like everyone else. The reason for Hamlet choosing to talk to the ghost is because of the fact that he takes the form of his father. Hamlet understands that the consequences of talking to this ghostly being without placing restrictions on the ghost could be damning; however, because of the shape the ghost takes, the image of his father, he says that he will talk to the ghost regardless of the potential consequences.
The supernatural agent (the ghost in Hamlet's father's form) plays a major role. If the ghostly agent had not been a part of Hamlet's world, he would probably not have found out that his father was murdered by his uncle. He would continue to be mournful of his father's death, but he wouldn't have become vengeful.