There are three kinds of people [who practice Buddhism]. Like all other beings, the lowest person wants happiness and not suffering or rebirth in the lower realms of existence, so he practices Buddhism to create the causes of rebirth in the human realm or in the heavenly realms of the gods. He does not have the power or the courage to leave worldly existence completely. He only wants the best parts of worldly existence; he wants to avoid the worst parts, and that is why he practices the Buddhist religion—to gain a higher rebirth.
The middling sort of person understands that the whole of worldly existence, no matter where one is born, is suffering by its nature, just as fire is hot by its nature. He wants to get out of it altogether and attain nirvana, the state that is entirely away from suffering.
The highest person realizes that just as he himself does not want to suffer and does want happiness, so also do all living beings have the same fears and wishes. He knows that since we have been born again and again from beginningless time in worldly existence, there is not a single sentient being who has not been our mother and father at one time or another. Since we are that close to all sentient beings, the best person is one who practices Buddhism in order to remove all these countless beings from suffering.
From Treasures of the Sakya Lineage: Teachings from the Masters by Migmar Tseten, page 41