Genèse 8:21

and the Lord said in His Word, I will not add again to curse the earth on account of the sin of the children of men; for the imagination of the heart of man is evil from his youth; neither will I add to destroy whatever liveth as I have done.

AND THE ETERNAL SAID IN HIS HEART. He did not reveal the matter to a prophet at that time, but on the day He commanded Moses concerning the writing of the Torah He revealed to him that as Noah brought his sacrifice it mounted pleasingly, and He decreed that He will no more smite every living thing. In this regard, I have already written of a secret hinted at by our Rabbis.
The sense of the expression, I will not again curse the ground any more for man’s sake is that they were punished because of him for if man had not sinned, they have been spared even though they had also corrupted their ways.

FOR THE IMAGINATION OF MAN’s HEART IS EVIL FROM HIS YOUTH. He ascribes merit to men because by their very creation they have an evil nature in their youthful days but not in their mature years. If so, for these two reasons, it is not proper to smite every living thing. The reason for the prefix mem [which signifies “from” in the word] min’urav (from his youth) is to indicate that the evil imagination is with men from the very beginning of their youth, just as the Rabbis have said: “From the moment he awakes to go forth from his mother’s womb the evil impulse is placed in him.” It is possible that the verse is saying that it is from youth — meaning, on account of youth — that the evil inclination is in man, for youth causes him to sin. And some say that it is as if it said “in his youth,” [min’urav being interpreted as if it were bin’urav]. Similarly we find ‘Miterem’ (Before) a stone was laid upon a stone in the temple of the Eternal, [where the word miterem is interpreted as beterem]; so too, This is the land which ye shall divide ‘minachlah’ (by lot) unto the tribes of Israel, [where minachlah is interpreted as benachlah].

את ריח הניחוח, proving that at that time all these species of ritually pure animals were fit as sacrifices.

ויאמר ה' אל לבו, what G’d said to Himself at this time He did not reveal to Noach and his sons until they had accepted His commandments and He had established a covenant with them.

כי יצר לב האדם רע, seeing that after the deluge the climatic conditions on the earth became considerably less favourable, people did not develop as much intellectually and at such an early age as prior to the deluge. Before the deluge, the evil urge, and the urge to act wisely and not self-destructively, had been in balance, now the evil urge was active already in their earliest youth and was not balanced by the presence of an active יצר טוב. [This helps us understand why G’d appeared to be more indulgent of human weakness after the deluge instead of the reverse. Ed.]

'וירח ה, the Torah uses a figure of speech to facilitate our understanding of G’d’s reaction, seeing that G’d has no nose and therefore cannot smell in the accepted sense of the word. When in Psalms 50,13 the psalmist speaks of G’d asking if He would “eat the flesh of bulls, or drink the blood of he goats,” this is also a figure of speech, of course. David most certainly did not suggest that such a thing were possible in the literal meaning of the words. The meaning of such phrases is that the offerings were pleasing to G’d as if, if we had been speaking of human beings, they smelled good and tasted good. The proof that G’d had accepted the offering and was well disposed toward all His creatures was the fire which descended from heaven and consumed the offerings. This is what David referred to in Psalms 104,31 when he described G’d as ישמח ה' במעשיו, “that the Lord took pleasure in His creatures,” as opposed to Genesis 6,7 when we were told ויתעצב ה' אל לבו, “that the Lord was saddened in His heart.”

את ריח הניחוח, a noun belonging to the same category as ניצוץ in Isaiah 1,31 meaning that Noach had succeeded in calming down G’d’s anger at the world, (His creatures).

ויאמר ה' אל לבו, He foresaw in His mind that He would not again have to wreak wholesale destruction on earth seeing that it would not happen again that the majority of the human species would be as depraved as those before the deluge. The report of what happened during the generation of the deluge was handed down from generation to generation and served as a warning against man again becoming as corrupt as at that time. G’d would not again punish the community at large on account of depraved individuals, but He would punish all the individuals who are wicked, as He demonstrated when He destroyed Sodom and its satellites.

בעבור האדם, on account of the wicked people among mankind; rather those will be killed so that the earth will be at the disposal of the good people. If small parts of the earth would indeed be destroyed as happened in the case of Sodom and that part of the Jordan valley, this cannot be considered as a destruction of the earth. Such destructions will be understood as the destruction of that particular population including the earth they had dwelled on.