Mark 5:18

That he might be with him; i.e. as one of his disciples. St. Ambrose says Christ did not grant his request, lest they might think that he sought to be glorified by men, in having always in his company a man out of whom he had cast so many devils. Christ himself seems to give us another reason, that the man might go, and publish in his own country the miracles done by Jesus. (Witham)