עֲשָׂרָה דוֹרוֹת מֵאָדָם וְעַד נֹחַ, לְהוֹדִיעַ כַּמָּה אֶרֶךְ אַפַּיִם לְפָנָיו, שֶׁכָּל הַדּוֹרוֹת הָיוּ מַכְעִיסִין וּבָאִין עַד שֶׁהֵבִיא עֲלֵיהֶם אֶת מֵי הַמַּבּוּל. עֲשָׂרָה דוֹרוֹת מִנֹּחַ וְעַד אַבְרָהָם, לְהוֹדִיעַ כַּמָּה אֶרֶךְ אַפַּיִם לְפָנָיו, שֶׁכָּל הַדּוֹרוֹת הָיוּ מַכְעִיסִין וּבָאִין, עַד שֶׁבָּא אַבְרָהָם וְקִבֵּל עָלָיו שְׂכַר כֻּלָּם:
Ten generations from Adam until Noach. To apprise us how long-suffering He is. For all of these generations they kept on angering Him — until He brought upon them the waters of the flood. [You, too, do not wonder that He has been long-suffering to the idolators all of these years that they have been subjugating His children, for He was even more so to the idolators of the (ten) generations from Adam until Noach — until they were destroyed in the flood.] Ten generations from Noach until Abraham. To apprise us how long-suffering He is. For all of these generations they kept on angering Him — until Abraham came and took upon himself the reward of all. [He did good deeds corresponding to what all of them should have done — wherefore they were all rescued in his merit. And just as he took upon himself the yoke of mitzvoth in this world over and against all of them, so did he receive for the world to come reward over and against all of them. For every man has two shares, one in Gan Eden; the other, in Gehinnom. If he is meritorious, he takes his share and that of his neighbor (the evildoer) in Gan Eden. If he is culpable, he takes his share and that of his neighbor (the tzaddik) in Gehinnom.]
Avot DeRabbi Natan
And why was it necessary to bring all those people into the world? This teaches you that even as all those generations continued to anger the Holy Blessed One, He did not bring the flood upon them because of the righteous and saintly among them. And some say that as long as Methuselah was alive, the flood did not come upon the world; and even when Methuselah died, it was still held off for another seven days, as it says (Genesis 7:10), “And it was seven days, [and the waters of the flood were on the earth].” What was happening during those seven days? These were the [seven] days of mourning for that righteous man, who had prevented the punishment. That is why it says, “And it was seven days.”
Another interpretation of, “And it was seven days”: This teaches that the Holy Blessed One fixed a specific time, after 120 years, hoping that they might repent. But they did not, and so it says, “And it was seven days” [i.e., the final seven days of the countdown].
Another interpretation of, “And it was seven days”: This teaches that the Holy Blessed One changed the order of the world [which was created in seven days] for them, and had the sun rise from the west and set in the east, hoping that maybe they would understand and become afraid and repent. But they did not, and so it says, “And it was seven days.”
Another interpretation: This teaches that the Holy Blessed One set a table for them, and gave them a taste of the World to Come, so that they would gather together and say to one another, “Oy, for we have lost all this goodness, and we have caused the destruction of our descendants (from the earth), as it says (Genesis 6:12), “And God saw the earth, and it had been destroyed.”