What is the Great Vehicle? What is the mode of procedure of the Bodhisattva path? We begin with the topic of the altruistic intention to achieve enlightenment in which one values others more than oneself. The Great Vehicle path requires the vast motivation of a Bodhisattva, who, not seeking just his or her welfare, takes on the burden of bringing about the welfare of all sentient beings. When a person generate this attitude, they enter within the Great Vehicle, and as long as it has not been generated, one cannot be counted among those of the Great Vehicle. This attitude really has great power; it, of course, is helpful for people practicing religion, but it also is helpful for those who are just concerned with the affairs of this lifetime. The root of happiness is altruism - the wish to be of service to others.
~The Dalai Lama