Lord Krishna in Bhagavad Gita details two prime reasons for sufferings by people world over.
1. Evolution supported by karma resulted in sufferings untold. This was part of cosmic design. In real life also if we wanted to extract metal from ore, we passed it through various mining machinery and processes that included burning in big ovens. Only then the sandwiched dirt could be removed around the metal.
For the period the ore containing metal remained in hot oven it was time for sufferings. The moment that treated ore was passed through a stream of cold water, it was time for experiencing happiness. It was through both sufferings and happiness life moved ahead in cosmic life cycle. No life can evolve in cosmic cycle in absence of sufferings or happiness.
2. When human beings indulged in bad karma, a vicious life cycle the resultant karma was negative. What if the person died! In the next manifestation, the new body manifested by soul atman, their destiny was already written. Because of bad deeds of past life, indulgence in negative karma in previous life one is bound to suffer in present life.
That was how law of karma operated. Even a single negative thought precipitates into an act of karma. To improve our present and also future, our indulgence in good karma, positive karma in present life is necessitated.
We just could not continue indulging in vices and expect better results thereof. Law of karma says, we reap what we sow nothing more or less. None in this world can avoid inscrutable karma.
Karma is that inherent principle of life around which rotates doctrine of evolution and entire progress in the manifest physical world visible all around.
To give an example, due to atrocities committed by Hitler on Jews in World War 2 he would be directly punished by God. So much so that in next manifestation the soul atman of Hitler would manifest a much lower form of life probably an animal form or even an insect.