A. The Brief Teaching Of The Essence

Why do these lives of wandering in the sufferings of samsara, each with its own appearances of joy and sorrow appear? They occur because of karma, due to cultivating it, it is explained:

Thus, samsara’s heights and depths of pleasure and pain
Arise from former accumulations of our karma.
That is how it was taught by the Sage, the Buddha.

From the different conditions of beings, different fruitions of their associated karma exist. With many kinds of experience, happiness and sorrow ripen. The Hundred Actions says:

E ma ho! Karma comes from the world.
Joy and sorrow are a painting produced by karma.
When conditions gather, karma is arising.
Happiness and suffering are produced by karma.

Also it says:

Karmas over the time of a hundred kalpas
Are never lost, but accumulate with time.
Once embodied beings have acquired them
The ripening of their fruition is assured.

The White Lotus (padma dkar po) says:

Karma is like a painter who produces everything.
Karmic patterns are choreography for a dancer.

The Gathering the Accumulations of Enlightenment (byang chub tshogs ’du ba) says:

Having, as well as being without, the three-fold kleshas
Are established according to merit and karma of liberation.
Because of mind and karma, in accord with the causes of beings,
There is a great assembly of karmas that are like seeds.