At first, a person who is being challenged by serious illness may not intellectually grasp the reason for thanking Hashem for the illness. If despite his lack of understanding, he persists in thanking Hashem simply out of Faith, he will eventually understand - on a deeper level- why the illness is intrinsically good.
The following miracle illustrates the above:
A young man from our yeshiva was told by a doctor that he had a fast-growing cancer in his reproductive organs and stomach. The doctor told him that if he didn't receive chemotherapy and radiation immediately, that he would be gone in three months.
The young man went straight to see Rav Arush who was already waiting for him in the doorway. He told Rav Arush the doctor's ominous news. Rav Arush told him that he didn't need any treatment. Instead, Rav Arush advised him to go to the field and thank Hashem for his illness every day. The young man listened to the Rebbe and has not had a single treatment - that was three and half years ago.
If you're interested, you can find him on the second floor of our yeshiva building. He is gratefully learning Torah like a Lion every day. (story courtesy of Rav Dror Moshe Coussoto)
Have a Great Week!
Dr. Zev Ballen