Assu Sutta - Discourse on Tears. Sutta Nipata 15.3

Who can deny the amount of suffering in life? Those who say Buddhism is pessimistic are in self denial. Over time, our minds tend to forget the pain we feel. This is its defence against a bitter reality of life, otherwise we would not be able to take the stress of repeated suffering. But when we recognise the reality of suffering, not only in this life, but in countless lives, we will be motivated to find a cure.

"Long have you (repeatedly) experienced the death of a mother... the death of a father... the death of a brother... the death of a sister... the death of a son... the death of a daughter... loss with regard to relatives... loss with regard to wealth... loss with regard to disease. The tears you have shed over loss with regard to disease while transmigrating & wandering this long, long time — crying & weeping from being joined with what is displeasing, being separated from what is pleasing — are greater than the water in the four great oceans.

Why is that? From an inconstruable beginning comes transmigration. A beginning point is not evident, though beings hindered by ignorance and fettered by craving are transmigrating & wandering on. Long have you thus experienced stress, experienced pain, experienced loss, swelling the cemeteries — enough to become disenchanted with all fabricated things, enough to become dispassionate, enough to be released."