A. The brief teaching of the essence.
(i.e. All happiness and suffering come from the mind, from self-conditioning, from accumulating good or bad karma. The message here is that we are conditioned by our past choices, but still have enough freedom to see through it. No total freedom, no total determinism. That is the introductory model; and its problem is "How is individual karma transmitted between lives? What is the support? Is karma individual, or universal?")
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:
Thus, Samsára’s heights and depths of pleasure and pain
Arise from former accumulations of our karma.
That is how it has been taught by the Sage, the Buddha.
(i.e. Our conditions, our happiness and suffering are not causeless, or the fruits of chance, or decided by an omnipotent god. There is a reason why some people are happy, while some others are suffering a lot. If we observe carefully our actions and their effects we will understand that there is logic of cause and effect. Then we will understand that our own happiness and suffering are caused by our own past actions.)
(i.e. Saleyyaka Sutta - The Brahmins of Sala, MN41:
4. When they were seated, they said to the Blessed One: "Master Gotama, what is the reason, what is the condition, why some beings here, on the dissolution of the body, after death, reappear in states of deprivation, in an unhappy destination, in perdition, even in hell; and what is the reason, what is the condition, why some beings here, on the dissolution of the body, after death, reappear in a happy destination, even in the heavenly world?"
5. "Householders, it is by reason of conduct not in accordance with the Dhamma, by reason of unrighteous conduct, that beings here on the dissolution of the body, after death, reappear in states of deprivation, in an unhappy destination, in perdition, even in hell. It is by reason of conduct in accordance with the Dhamma, by reason of righteous conduct, that some beings here on the dissolution of the body, after death, reappear in a happy destination, even in the heavenly world.")
From the different conditions of beings, different fruitions of their associated karma exist. Many kinds of connection with their 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.
The assembly of conditions arises karmically.
Happiness and suffering are produced by karma.