Why does Teddy Sands reject the Purnell Plan in the novel, The Martian?

When Teddy is given the choice of using a plan that involves sending the Ares crew back – known as the Purnell plan – or the Iris 2 plan to make a pre-supply drop at Ares 3 that involves no crew members, he opts for the latter. He would rather risk one crew member versus the death of six astronauts.