This Book Is Recommended Preliminary Reading for the July Kalachakra Teachings.
"The Buddhas have already achieved all their own goals, but remain in the cycle of existence for as long as there are sentient beings. This is because they possess great compassion. They also do not enter the immensely blissful abode of nirvana like the Hearers. Considering the interests of sentient beings first, they abandon the peaceful abode of nirvana as if it were a burning iron house. Therefore, great compassion alone is the unavoidable cause of the non-abiding nirvana of the Buddha." --Kamalashila
Compassion's importance cannot be overemphasized. Chandrakirti paid rich tribute to compassion, saying that it was essential in the initial, intermediate, and final stages of the path to enlightenment.
Initially, the awakening mind of bodhichitta is generated with compassion as the root, or basis. Practice of the six perfections and so forth is essential if a Bodhisattva is to attain the final goal.
In the intermediate stage, compassion is equally relevant. Even after enlightenment, it is compassion that induces the Buddhas not to abide in the blissful state of complacent nirvana. It is the motivating force enabling the Buddhas to enter non-abiding nirvana and actualize the Truth Body, which represents fulfillment of your own purpose, and the Form Body, which represents fulfillment of the needs of others. Thus, by the power of compassion, Buddhas serve the interests of sentient beings without interruption for as long as space exists. This shows that the awakening mind of bodhichitta remains crucial even after achieving the final destination. (p.44)
--from Stages of Meditation,
book by His Holiness the Dalai Lama,
published by Snow Lion Publications