The companions shape and make the Doctor who he is. Each one changes him and affects him in a different way. They each teach him wot it means to be human, as well as wot it means to be a Time Lord, a defender, a champion. They are his soul, his values, his friends and loved ones. The show is so much more than just about the Doctor and his adventures, it's about wot each companion brings to the adventure and wot they teach him about who he is and could be.