All the Buddhas in the past including Shakyamuni Buddha, after having aroused bodhicitta, had vowed to deliver all sentient beings to liberation before attaining Buddhahood themselves. Yet they have already attained Buddhahood while we still remain in samsara. Did all the Buddhas break their vows? No. The Buddhas’ vows were made out of their deep compassion for the sentient beings. By the supreme power of the grand vows, they were able to swiftly attain Buddhahood. Whereas the thought that is constantly being turned over in our minds is usually just our own welfare, which explains why we are still struggling in samsara as yet. This clearly demonstrates that the key to attaining Buddhahood rests squarely on nothing but one’s altruistic aspiration.

- Quote from The Right View, "Liberating Living Beings"