Why do Huck and Jim run away together in The Adventures of Huckleberry Finn?

Actually, Huck and Jim don't decide to run away together.
Both run away separately, and then they meet up by chance on the same island, Jackson
Island, when Huck discovers that somebody else is camping out on the island as well.
This occurs in Chapter Eight, as Jim tells Huck that he ran away from Miss Watson
because she was going to sell him. Huck of course ran away, faking his own death, so
that he could escape from his father, who was abusing him and locked him up. It is only
when Huck disguises himself as a girl to find out news of what is going on that he
discovers that the fire they lit had been seen from the mainland and that men are
planning to visit Jackson Island to hunt for Jim and collect the reward money for a
runaway slave. It is then that Jim and Huck run away together.