Imam Khomeini explains the matter through his theological works as following:
God does not want man to be afflicted with illnesses unaccompanied by pain, for when an illness is accompanied by pain, it forces man to seek a cure, to consult a doctor or go to hospital, but an unfelt illness without pain is more dangerous. By the time one becomes aware of it, it is too late.
If mental illnesses were accompanied by pain, this would be something for which to be thankful. Ultimately, man would be forced to find a cure or a remedy.
But what can be done about such dangerous diseases for which there is no pain? The illnesses of arrogance and selfishness are without pain.
Other sins corrupt the heart and the spirit without causing any pain. Not only are these illnesses unaccompanied by pain, but they also bring apparent pleasure.
Meetings and sessions of backbiting are very warm and sweet! Love of the self and love of the world, which are the roots of all sins, are pleasurable.
One who is afflicted with dropsy may die from water, but yet enjoy drinking it until his last breath.
Naturally, if one gets pleasure from an illness, and it also has no pain, he will not seek any cure for it.
However much he is warned that it is fatal, he will not believe it.
If someone is afflicted with the illnesses of hedonism and worshipping the world, and his heart is filled with love of the world, he will grow weary of all else but the world and what is in it, Imam further explained.
Allah forbid, he will become an enemy of God, the servants of God, the divine prophets and awliya, and the angels of Allah.
He will have sense of hatred and loathing for them, and when the angels come at the command of the Glorious God to take his soul, he will have a feeling of repulsion and abhorrence, for he will see that God and the angels of Allah want to separate him from his beloved (the world and worldly things).
It is possible that he will leave the world with hostility and enmity toward the Presence of the Exalted Truth (God).
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