When the waters of the Mabul finally receded and Noach could already venture out of the teivah, he erected a mizbeach and sacrificed korbonos from the kosher animals and fowl that had been with him in the teivah.
The possuk thereafter says וַיָּרַח ה' אֶת רֵיחַ הַנִּיחֹחַ וַיֹּאמֶר ה' אֶל לִבּוֹ לֹא אֹסִף לְקַלֵּל עוֹד אֶת הָאֲדָמָה בַּעֲבוּר הָאָדָם וגו' and Hashem, so to speak, 'smelled' the pleasant smell of the offerings and He said 'I will no longer curse the earth due to the sins of man...' Here, the Torah provides us with the reason for why Hashem will not smite earth again to such a degree, where the possuk continues כִּי יֵצֶר לֵב הָאָדָם רַע מִנְּעֻרָיו for man possesses an evil inclination already from his youth.
The Ramban explains that Hashem is מְלַמֵּד זְכוּת, sees merit for the benefit of mankind, and does not punish them so severely, in exact accordance to their sins, because they are born with an extremely powerful evil inclination which constantly incites them to sin.
The אור החיים הקדוש explains that because man is born with an evil inclination, Hashem promised not to destroy the earth and make humanity extinct. He quotes a Mishnah in בבא קמא (לט.) which deals with the law of the שׁוֹר הָאִיצְטַדִין, the theatre ox that was trained to fight for the entertainment of others. When this ox causes damage, the owner is absolved from responsibility because the ox was trained to damage. Hence, he cannot be expected to refrain from violence. So too, man is predisposed to evil already from a young age. Therefore, he is somewhat absolved from wrongdoing.
The Opposing Force of Good
However, the Ohr Hachaim continues to tell us, that much more is expected of man, for he is far greater than a beast from the field, using the words of the possuk וּמוֹתַר הָאָדָם מִן הַבְּהֵמָה אָיִן (קהלת ג יט) - man is more than the animal. Indeed, Hashem also created us with a Yetzer Hatov, an ability to choose to do what is right and good. We have the seichel, the unique wisdom, and understanding granted to man, to choose the correct path. Therefore, we are expected to constantly strengthen ourselves and rise above our evil inclination.
Thus, although Hashem promised not to completely destroy the world or humanity as a result of their sins, they will still bear responsibility for their actions and be punished accordingly. For although we are רַע מִנְּעֻרָיו, evil from youth, we also possess the ability to choose good; therefore, the Ohr Hachaim cautions us to disgrace evil and rather follow the path of truth.
(רמב"ן ואוה"ח ח כא ד"ה כי יצר)