Why do you think Jesus did not permit the man he healed from demons to accompany him on his journey?

Dennis R MacDonald (The Homeric Epics and the Gospel of Mark) believes that Mark's Gospel is impressive for the apparent number of direct and indirect borrowings from Homer. One or two could be a coincidence, while several would be significant evidence of a relationship, but MacDonald presents numerous examples.

Mark's story of the voyage across the Sea of Galilee and the demonic possession of the swine of the Gadarenes is a particular case wherein MacDonald demonstrates many parallels with Homer. Circe had turned a large number of Odysseus' men into swine, while Jesus cast the many demons into swine, a totally superfluous step in destroying them. When Odysseus was on his boat and leaving, he told Polyphemus to tell everyone what the great Odysseus had done to him, and similarly when Jesus was on his boat and leaving, he told the demoniac to tell everyone what 'the Lord' had done to him. Finally, the giant asked Odysseus to return, but Odysseus refused the request and sailed away with his crew, just as the healed man asked Jesus to allow him to go with him on the ship, but Jesus refused the request and sailed away with his disciples.

Looking a MacDonald's fuller comparison of the two accounts, there is very good reason to believe that the story in Mark's Gospel, and copied in the other gospels, really was inspired by Homer's account. Both stories end with the hero refusing the former wild man's request, and sailing away.