"The perfection of enlightenment is Buddhahood. Buddhas are reincarnated solely to help others, since they themselves have already achieved the highest of all levels. They are not reincarnated through any active volition of their own; such an active mental process has no place in nirvana. They are reincarnated, rather, as a result of this same innate wish to help others that allowed them to achieve Buddhahood."
The Dalai Lama of Tibet