All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If you are referring to the reason why people sin, then there are many reasons for this, among which are the following:
1- One’s ignorance of Allaah and of His Names and Attributes, because if one is more knowledgeable of Allaah, he would be more fearful of Him; it is for this reason that the Prophet, sallallaahu ‘alayhi wa sallam, said: “I am the most pious amongst you and the most knowledgeable of Allaah amongst you.” [Al-Bukhari] This Hadeeth was narrated by ‘Aa’ishah .
2- Being deceived in respect to the forbearance of Allaah and Him giving respite to His Slaves and Him not hastening the punishment for them; Allaah Says (what means): {And if Allaah were to impose blame on the people for what they have earned, He would not leave upon it [i.e. the earth] any creature. But He defers them for a specified term. And when their time comes, and then indeed Allaah has ever been, of His servants, Seeing.}[Quran 35:45]
3- Weakness of faith and the heedlessness that afflicts the hearts; Allaah Says (what means): {No! Rather, the stain has covered their hearts of that which they were earning.}[Quran 83:14] Allaah also Says (what means): {I will turn away from My signs those who are arrogant upon the earth without right; and if they should see every sign, they will not believe in it. And if they see the way of consciousness, they will not adopt it as a way; but if they see the way of error, they will adopt it as a way. That is because they have denied Our signs and they were heedless of them.}[Quran 7:146]
4- Having long hope (in life), following desires, having a hardened heart, and being lured by this world and its adornments. Allaah Says (what means): {Has the time not come for those who have believed that their hearts should become humbly submissive at the remembrance of Allaah and what has come down of the truth? And let them not be like those who were given the Scripture before, and a long period passed over them, so their hearts hardened; and many of them are defiantly disobedient.}[Quran 57:16] Allaah also Says (what means): {…and do not follow [your own] desire, as it will lead you astray from the way of Allaah.}[Quran 38:26] Allaah further Says (what means): {So let not the worldly life delude you.}[Quran 31:33]
5- The devil: He is the master of temptation and the pivot of evil, as Allaah Says on his tongue (what means): {[Satan] said, “Because You have put me in error, I will surely sit in wait for them [i.e. mankind] on Your straight path. Then I will come to them from before them and from behind them and on their right and on their left, and You will not find most of them grateful [to You].”}[Quran 7:16-17]
Finally, we hope that we have answered your question, and if you mean something else, then please clarify it to us so that we may answer your question precisely.
Allaah Knows best.