All those who suffer in the world do so because of their desire for their own happiness

All those who suffer in the world do so because of their desire for their own happiness. All those happy in the world are so because of their desire for happiness for their happiness of others.

Shantideva, Bodhicarayavatara ("Entering into the Bodhisattva Way of Life" - translated by Kate Crosby and Andrew. Skilton, 1996).