A monotonous life in ease and comfort would make the man forget his duty in this world. Sicknesses or disasters make him remember that duty.
The man was created to worship his Lord. This is his sole duty, indeed. However, we, the sons of Adam, easily forget that single duty. We fail to remember that this worldly life has an end. We stop thinking about the Hereafter, our eternal life; but make plans for the world, our mortal life. We put out of our minds that the eternal happiness is achieved through the good deeds, which we must perform in this world. Because of all these, we do not want to go through any misfortunes, which cause us discomfort in the life.
Illness, for instance, is a real trouble for us; because it makes us suffer and spoils our pleasure. We complain and do not want illnesses, misfortunes or calamities in this beautiful world so that we can live a happy life (again forgetting that it is transitory).
We do not want to remember that we are mortal. If there were no illness, misfortune or calamity here in this world, we will supposedly live a happy worldly life; but, on the other hand, we would totally forget our duties, which would cost our eternal happiness in the next. Many thanks to our Lord that this life is not like that!
Although we may not realize it, misfortunes save our lives. They warn and remind us that we are not here in this world forever; and that we are mortal. This makes us remember that we have a duty to our Creator.
Therefore, we can say that anything that helps us to do our real duty and perform our worships is accepted as good. Anything that prevents us to carry out our duty is regarded as bad.
This, however, depends on individuals. Something beneficial for a person can be something harmful for me. Or a good thing for me can be a bad thing for someone else. For instance, wealth might be a good means of worshipping Allah (swt) for one person. He can spend his money in the way of his Lord. But another person might go astray because of the same wealth. Intelligence, for example, can make someone serve the people in a great way with which Allah (swt) is pleased; but it may cause someone else to work for irreligious ideologies.
It is same for health. We see illness something bad, but it can be good for some of us. Wherever we see a patient, for example, they are closer to Allah (swt). Their words and deeds are mostly about doing their duty to Allah (swt). A healthy person, on the other hand, may neglect his worship, even does bad deeds or even spends his entire life in heedlessness.
This worldly life is a place of testing and worship. Considering that this is the place to serve our Creator, illnesses and misfortunes (as long as they do not affect faith and are endured patiently) help us to worship Allah (swt) properly. They even increase our sincerity. Therefore, we should see sicknesses as remedies in fact.
Although we wish a worldly life without any illness or misfortune, they can be actually helpers to us to pass the test of this life and gain eternal happiness in the next.