How can a Muslim regain his morale, ambition, and contentment with Allah's divine decree and destiny after being afflicted with an illness that caused him to lose these, and is affliction with illness a sign of love or punishment from Allah, and is supplication (du'a) specific to incurable diseases or does it include all diseases, and how can one respond to those who claim that their supplication is not answered?

It cannot be definitively stated that tribulations are caused by sins. However, it is advisable to address the matter with patience, increased seeking of forgiveness (istighfar), supplication (du'a), and having a good opinion of Allah, for a tribulation may be a punishment or a test. It has been reported in a hadith that when a believer endures hardship with patience, it is good for him. You should remember the many blessings of Allah, as the tribulation is nothing in comparison to them. Supplication benefits both what has descended and what has not descended, and it is the key to all good. One must be cautious of seeking a hasty response, for supplication is an act of worship for which you are rewarded whether the answer is granted or not, as Allah either stores good for you or averts tribulation from you.