In The Name of Allah, The Most Merciful, The Bestower of Mercy.
Imam As-Sadi [may Allah have mercy upon him] said:
The Exalted (Allah) has revealed the wisdom underlying inheritance, the transfer of wealth after death, and how to give it to the heirs. Allah said: [لا تدرون أيهم أقرب لكم نفعا – You know not which of them, whether your parents or your children, are nearest to you in benefit]. [Surah An-Nisaa. Ayah 11]
So, Allah fixed these shares based on His perfect knowledge of what is most advantageous, what a person usually loves in terms of who his wealth should reach, and who is more deserving of his kind treatment and favour, and arranged it in such a way that sound minds would attest to its excellence. And if the matter had been left to people’s opinions, desires, and wants, it would have led to flaws, disorder, poor decisions, the departure of order, and ultimately chaos. Allah commanded a person to direct some of his wealth in causes of righteousness and piety that will benefit him in the Afterlife and limited that to a third or less than that of a heir, lest the affairs which Allah has secured for the people become an issue of self-interest at the time of departure from this world for those people who are deficient in intellect and religion. (footnote a) (1)
Footnote a: In order to realize the beauty of this subject matter and its details, then must listen to lessons on these links by Shaikh Abu Hakeem and Shaikh Uways At-Taweel [may Allaah preserve them:
[Ref 1: Ad-Durra Al-Mukhtasar Fee Mahaasin Deen Al-Islaamiy. pages 27-28]