Why Allah swt allows us to get sick?

In the name of God, The Most Gracious, The Most Merciful

I visited Allah's Apostle  while he was suffering from a high fever. I touched him with my hand and said, "O Allah's Apostle! You have a high fever." Allah's Apostle  said, "Yes, I have as much fever as two men of you have." I said, "Is it because you will get a double reward?" Allah's Apostle  said, "Yes, no Muslim is afflicted with harm because of sickness or some other inconvenience, but that Allah will remove his sins for him as a tree sheds its leaves."
[Hadith Bukhari 7:564]

Abu Sa'id al-Khudri r.a. reported that the Prophet  said: Whenever a Muslim is afflicted with a hardship, sickness, sadness, worry, harm, or depression --even a thorn's prick, Allah expiates his sins because of it.
[Hadis Bukhari and Muslim]

"Expect good, because Allah (s.w.t.) makes a believer's sickness an expiation (for his sins) and a period of repose.  As for a disbeliever falling sick, he is like a camel whose owner ties it and then lets it loose --it does not understand why it was tied, nor why it was freed."
[Recorded by Bukhari in al-Adab ul-Mufrad (no. 493), verified authentic by al-Albani]

The cure

Allah's Messenger  said: There is a remedy for every malady, and when the remedy is applied to the disease it is cured with the permission of Allah, the Exalted and Glorious.
[Hadith - Sahih Bukhari, Book 25, Number 5466, Narrated Jabir ibn Abdullah]

Yahya related to me from Malik from Zayd ibn Aslam that a man received a wound in the time of the Messenger of Allah . The blood clotted in the wound and the man called two men from the Banu Ammar tribe. They looked at it and claimed that the Messenger of Allah  said to them, "Which of you is the better doctor?" They said, "Is there any good in medicine, Messenger of Allah?" Zayd claimed that the Messenger of Allah  said, "The one who sent down the disease sent down the remedy."
[Hadith - Muwatta 50:12]

Allah's Messenger  said, "Fatihat al-Kitab contains healing for every disease."
[Hadith - Al-Tirmidhi #2170, Narrated AbdulMalik ibn Umayr]
Darimi and Bayhaqi, in Shu'ab al-Iman, transmitted it.



Severe Trials for those Firm in their Religion

When the Prophet  was asked which people suffered the greatest affliction, he replied, "The prophets, then those who come next to them, then those who come next to them. A man is afflicted in keeping his religion. If he is firm in his religion his trial is severe, but if there is weakness in his religion it is made light for him, and it continues like that till he walks on the earth having no sin."
[Hadith - Al-Tirmidhi 1562, Narrated Sa'd]
[Tirmidhi, Ibn Majah and Darimi transmitted it, and Tirmidhi said this is a hasan sahih tradition.]



Rewards for those Afflicted

The ailment is intensified for the righteous.  Whenever a believer is afflicted with a hardship, be it a thorn or more, a sin is taken off him because of it, and he is elevated on level (in Jannah [Paradise]).
[Recorded by Ahmad , verified authentic by al-Hakim, ath-Thahabi, and al-Albani]

A traveller's or sick person's deeds are recorded in accordance with what he used to do when he was resident or well.
[Hadith - Al-Bukhari]



Endurance

The Prophet  stated that Allah, who is Blessed and Exalted, said "Son of Adam, if you show endurance and seek your reward from me in the first affliction, I shall be pleased with no lesser reward than Paradise for you."
[Hadith - Al-Tirmidhi, Narrated AbuUmamah. Ibn Majah transmitted it.]

The Prophet of Allah, upon him be peace, said, "Do not say, 'I feel rotten*.' Instead, say, 'I do not feel well."
*or any expression that involves corruption of any type.
[Hadith - Sahih Bukhari, Muslim, and Nasa'i, Narrated 'A'ishah]



Kindness to the sick

Allah's Messenger  said, "When you go in to visit an invalid, express a hope that he will live long. That will not avert anything, but it will comfort him."
[Hadith - Mishkat, Narrated Abu Sa'id]

Allah's Apostle  said, "You see the believers as regards their being merciful among themselves and showing love among themselves and being kind, resembling one body, so that, if any part of the body is not well then the whole body shares the sleeplessness (insomnia) and fever with it."
[Hadith - Sahih Bukhari, Volume 8, Book 73, Number 40, Narrated An-Nu'man bin Bashir]


Taking Medicine

The Prophet  said: Every disease has a medication: If the medication for a particular disease is found, it will be cured with Allah's (swt) permission.
[Hadith - Muslim, narrated Jabir Bin 'Abdillah]

Abu Bakr kissed (the forehead of) the Prophet  when he was dead. 'Aisha added: We put medicine in one side of his mouth but he started waving us not to insert the medicine into his mouth. We said, "He dislikes the medicine as a patient usually does." But when he came to his senses he said, "Did I not forbid you to put medicine (by force) in the side of my mouth?" We said, "We thought it was just because a patient usually dislikes medicine." He  said, "None of those who are in the house but will be forced to take medicine in the side of his mouth while I am watching, except Al-'Abbas, for he had not witnessed your deed."
[Hadith - Bukhari 7:610, Ibn 'Abbas and 'Aisha, ra.]


Read Qur'an when Sick

And We send down from the Qur'an that which is a healing and a mercy to those who believe (in Islamic Monotheism and act on it), and it increases the Zalimun (polytheists and wrong-doers) nothing but loss.
[Surah Al-Isra 17:82]

...

Note:

1. Do not jump to the conclusion that all sicknesses are punishments from Allah, subhana watala.

2. A woman may be treated by a male physician (including treatment or examination of private parts, etc.) only if a female physician cannot be obtained through all means possible, and it is recommended that a mahram male or trustworth muslim female be present to prevent the occurance of khulwah.

3. Males should avoid treatment by non-mahram female doctors or nurses, except when male doctors or nurses are not available.

4. Do not look down on a person who is sick, merely because of the sickness.

5. Performing ruqyah is permissible so long that it does not contain shirk or anything else haram.  But do not seek ruqyah, because that can prevent you from being among the seventy thousand who immediately enter Paradise.  You may, however, volunteer on your own and perform ruqyah over an individual if s/he has not requested it and still be eligible to be among the seventy thousand who immediatley enter Paradise. Allah knows best.

6. Count your blessings while you are sick, using the opportunity to seek forgiveness of sins and remaining patient during the affliction.

Source: Muttaqun.com