Ordinary people, unable to break loose from samsara, have no choice but to remain trapped in the cycle of rebirth. Whereas the bodhisattvas, no longer being bound by samsara, choose to remain because sentient beings only exist in samsara, not in nirvana. In order to deliver sentient beings from all suffering, the bodhisattvas must work from within, not out of, samsara. 

- Quote from The Right View, "The Four Noble Truths—the Path Out of Samsara"