The Cause of Distress
Diístress (Buddhist: dukkha), i.e. negatively experienced stress, is felt when a living being under-performs (Buddhist: under achieves).
Happiness (i.e. conístress (my word invention), i.e. positive stress, Buddhist: sukkha) is felt when a living being performs, i.e. achieves, ecstatic joy when it outperforms, i.e. becomes ecstatic.
In short, diístress (e.g. the stick) and conístress (e.g. the carrot) function as a living beingís Guide & Control mechanism.
Since, according to the Buddha Keynes, Ďin the long run we are all deadí, we all eventually underachieve. And that results in much diístress, felt as sorrow or suffering. In this sense the Sakya (i.e. Old) Buddha was right. However, diístress as such does not result, as he claimed, from transience and dependence (i.e. non self-ownership) as such but from the inability to overcome both transience (specifically decay) and dependence. This is achieved, albeit temporarily) by attaining quantum status, i.e. an order of 1, and which is experienced as a true self (or atta).
In the course of time, the Old Buddha changed his mind (i.e. when he must have remembered how he himself had attained samma-sambodhi, i.e. perfect awakening ≈ realization) and took a more anticipatory and pro-active view. Diístress is ended and conístress (i.e. happiness) is achieved through attainment, for instance, via the perfection (i.e. samma-sam (bodhi)) of a personal function or task, such as Ďawakening to the functions domain of distress)í or one of the means included in the Noble 8-fold Path. His Mahayana followers made much of the attainment (read: perfection, actually meaning whole satisfaction) mode of diístress avoidance.
Again, dukkha and sukkha are a Guide & Control system. The world (i.e. Samsara) is not a mass of distress. Distress is caused by failure to respond @ best (and better) to the world.