The Kedushas Levi points out in the name of his father that the possuk starts וַיָּרַח ה' אֶת רֵיחַ הַנִּיחֹחַ and Hashem 'smelled' the pleasant scent, and then continues with the reason of why Hashem will not destroy the world כִּי יֵצֶר לֵב הָאָדָם רַע מִנְּעֻרָיו. This appears to be somewhat perplexing, what was the reason for Hashem's promise?
He explained that because man is essentially born wicked, it therefore brings such great nachas to Hashem when he chooses to do good. Because he overcame his natural inclination, his inborn רע, therefore his self-sacrifice is a רֵיחַ נִיחֹחַ an extremely pleasant offering. Due to the nature of the struggle, the achievement is far greater!
(קדושת לוי פר' נח ד''ה וירח ה')
For a Greater Purpose
The Chofetz Chaim in his sefer Toras Habayis (פרק א) expounds upon this concept at length. He says that man's entire purpose is to overcome this natural instinct. This is the reason why he was created, and for this he will be justly rewarded. Because the natural world is so far removed from the holiness of the neshama therefore man's mission is so much more crucial, and is such an integral part of the תַּכְלִית הַבְּרִיאָה, purpose of Creation.
He presents a mashal which explains this concept. There was once a king who lived together with his son in a magnificent palace which lacked nothing. However, he sent his son to a faraway land where the cold was brutal, the food was abominable and the people were crude. While there, he had to wear coarse clothing to protect himself from the cold, he had to eat foods of inferior quality to which he was not accustomed, and he had to interact with the population who were far below his caliber. Whoever saw the drastic difference of his lifestyle while in this faraway land understood that surely his father must have sent him there on a crucial mission. For understandably he would only subject him to such harsh circumstances for an important goal, as the Chofetz Chaim says; לְתַכְלִית רָם וְנִשָּׂא for an elevated, greater purpose.