The only thing that comes close to a motive is provided by the character of Jim played by C. Thomas Howell. Being the only person ever to stand up to The Hitcher, he is subsequently tormented and played with for the rest of the film.
It's proof that you don't always need a deep character to make a piece compelling. They don't need complex motives to propel the story along. Sometimes nothing more than evil itself provides the perfect antagonist.