A few more from two books that happen to be next to the keyboard:
Anyone who wants to achieve the Way of enlightenment must drive forward the wheel of the Four Great Vows.
- Opening words of Hakuin's autobiography Wild Ivy
The strength of the vow [to practice] is founded on Great Compassion. Those who seek from selfish motives only attain to a shallow insight. A merchant, for example, striving for his own security, will be satisfied with but a small profit, and be proud of it. But he who wants to give everything cannot be satisfied with small gains. For this reason, the first of the Four Great Vows is to assist sentient beings. To see into one's true nature, to cut off the root of the afflicting passions, to learn all the Dharma-gates [teachings], to practise the way of the Bodhisattva and fully to ripen Compassion and Wisdom - this is the Buddha's Way. Truly, truly, Great Compassion is the origin and foundation of becoming Buddha.
When closely observing sentient beings, it appears that they always throw away the origin and chase after end-states; thus, much attached to all kinds of karma-producing activities, dying here and being born there, they revolve through the various stages of the Wheel of Becoming. The Five Signs of Decay of heavenly beings, the Eight Hardships of men, the states of hungry ghosts and of animals, the excruciating pains of the hells - just try with all your might to imagine these and feel them in your own heart.
Again, life after life, all sentient beings become fathers and mothers, are brothers and sisters, world after world. Considering this today, what a great debt of love we owe to each other! Reflecting on this, Great Compassion is bound to arise in the heart.
To state it concisely: by the power of the vow of Great Compassion all karmic obstacles disappear and all merit and virtue/strength are completed. No principle remains obscure, all ways are walked by it, no wisdom remains unattained, no virtue incomplete
The first requirement for trainees, therefore, is to let go of "I" and not to cling to their own advantage.
- Torei Enji, Shumon Mujintoron
As has been often discussed, Zen's approach is to directly point out by various means one's nature. Having this recognition is the entrance into Zen practice, and the beginning of the continuing practice to integrate one's recognition as realization by which all good qualities are manifested. It is also the root of true compassion.
In the past, someone going to study with a Zen teacher would likely have a basic grounding in Buddhist teachings, and so it may seem that Zen teaching/texts at times stress details of practice rather than motivation, precepts and so on.
An authoritative and basic text on the Rinzai side is Torei's book mentioned above (two available translations are titled "The Inexhaustible Lamp" and "The Undying Lamp of Zen"). The second chapter of this text is dedicated to the importance of bodhicitta, vows, faith and compassion as the foundations of Zen practice. It contains many pages of the kind of thing quoted here.