The reasoning behind the Sirens' actions were never truly explained though in Greek mythology (like many mythologies) the supernatural world often had little explaination and the spirits and gods could torment mortals as they saw fit - Sirens were not gods but they were sometimes said to be related to the great sea-god known as Poseidon. In Jason and the Argonauts, Jason ordered Orpheus, the great musician, to play his lyre so beautifully that the song of the Sirens would have no power over the crew. They made it safely past thanks to Orpheus.