There are generally three reasons why adversity occurs. First of all, as noted, we are all subject to natural and physical laws. Natural laws, such as the rising of the sun and the ebb and flow of the tide, bring beauty and order to the world and make life possible. But because we are subject to such laws, natural disasters will sometimes occur. The awesome power of nature, such as in a strong typhoon, can result in devastation, loss of income, loss of opportunity and death.
And yet from floods, rubble, and ash can come a new understanding of life which we would never have understood by other means. For instance, the Japanese have produced some of the sturdiest and most advanced structures in the world. Why? Because of the numerous earthquakes that devastate the island country. Such experiences can help us learn, grow, and reach out to others better.
Second, since we are not perfect, we all make mistakes and must face the consequences. Some consequences, such as missing the bus because you woke up late, are minor. The effect of other acts, such as stealing money, can be enormous and can cause pain, not only to you but to others as well. It can be painful to learn from our mistakes. But from such experiences we can develop greater wisdom and compassion.
Lastly, (and this is perhaps the most important), suffering can humble us and make us more spiritually-minded. To some, this means becoming closer to God or a Higher Force. To others, it is a way to see life not as a material wonderland but as a spiritual journey towards perfection and bliss. The great spiritual lights of mankind, such as Jesus Christ, Muhammad, Moses and the Buddha, all underwent intense suffering before they became sources of enlightenment and salvation to those who followed them.