O impatient sick person! The fact that those who come to this world continuously depart, and the young grow old, and human beings perpetually revolve amid death and separation, testifies that they did not come to this world to enjoy themselves and receive pleasure.
Also, while human beings are the most perfect, the most elevated of living beings and the best endowed in regard to members and faculties, the thought of past pleasures and future pains makes them live a mere woeful, troublesome life, lower than the animals.
This means that human beings have not come to this world purely to live in a fine manner and pass their lives in ease and pleasure. Rather, by holding a vast capital, they have come here to work and do trade for the happiness of an eternal and everlasting life. The capital given to them is life. If there is no illness, good health and well-being cause heedlessness and show the world to be pleasant, and make the hereafter forgotten. They stay clear of remembering death and the grave; and cause the capital of life to be wasted on trivialities. As for illness, it suddenly opens the eyes, and says to the body: “You are not immortal. You are not aimless. You have a duty. Give up your pride, think of the One Who created you. Know that you will enter the grave, so prepare yourself for it!”
That is why, from this point of view, illness is an awakening guide and an advisor that never deceives. It should not be complained about, it should be thanked for in this respect. And if it becomes too much to bear, patience should be sought to endure it.
Source: Twenty Fifth Flash: Third Remedy.