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Detailed Answer
If what you mean is why one isn't permitted to deprive himself of the blessing of life, the answer is:
1- The mind and nature of every person understands that destroying any blessing of this world is inappropriate; this matter is so clear to the extent that all people, regardless of their ages and religion all understand it. It is because of this that different organizations are established in the world today; organizations responsible for protecting animals and the environment and the like. Also, there are organizations responsible for controlling and protecting all forms of life in this world who pass different laws in order to do so. Now that the intellect and the nature of man can't accept the smallest god-given blessings being wasted and destroyed, how can one accept the greatest blessing of Allah (swt), which is life, to be wasted?
This little introduction shows that if one ever wishes to do so, he has strayed from the natural path of the mind and his own nature and can even be considered to be a sick person who needs treatment. There are different centers today who can cure this sickness using psychological methods.
2- The clear religion of Islam, whose guidelines are in accordance with man's nature and the mind and truth, has prohibited suicide and sees it as a haram act.
3- According to Islamic ideology and Quranic teachings, Allah (swt) is the absolute owner of the entire universe[1] and Allah (swt) has bestowed these blessings upon us so we utilize them for the purpose of our own perfection. Therefore, misusing and wasting these blessings isn't to their true owner's consent and the mind tells us that we have to use things we have been entrusted with in a way that their owners are content about, not in any other way.
4- Another more important point is that one of the other reasons why suicide is haram is that not only does Islam want to protect the lives of the individual, but it also wants to protect the life of society as well.
What is meant is that when Allah (swt) forbids one to kill himself (whom he thinks belongs to himself), he certainly won't have the permission to kill others; the prohibition of suicide prevents murder from spreading in society.
Usually people suffering from psychological problems commit suicide and naturally; one can't expect those who have reached such a level of psychological illness that drives them to not having mercy on and killing themselves to have mercy on others and not do the same to them. If Islam was to allow suicide, there are chances that those who had reached a level in which they would consider it as an option for themselves, would first attempt to take the lives of others and Islam would have worked against mankind in this case (by issuing the permissibility of suicide).
This is why Islam has counted suicide as one of the greatest sins and has said that those who commit such a sin will be severely punished.[2]
[1] "و لله ملک السموات و الارض" Ale-Imran:189.
[2] Imam Sadiq (as) says: "Whosoever purposely commits suicide, will remain in the hellfire forever." Man-La Yahduruhul-Faqih, vol. 4, pg. 95.
source : www.islamquest.net
