“Sentient beings are strange, although wishing happiness they avoid its causes. Although wishing to avoid suffering they constantly create its causes.” -Shantideva

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What is there need to say more?

The childish work for their own benefit,

The buddhas work for the benefit of others.

Just look at the difference between them.

If I do not exchange my happiness

For the suffering of others,

I shall not attain the state of buddhahood

And even in samsara I shall have no joy.

The source of all misery on the world

Lies in thinking of oneself;

The source  of all happiness

Lies in thinking of others.

-Shantideva