This is another hard saying that is not made particularly clearer or more certain in other translations. The root of the Hebrew word that “strive” translates, דִּין or din, can mean variously judge, vindicate, or abide (at least).The context is essential to fixing the sense. So note that it is because man is flesh that the Lord’s spirit will not “strive” with man, and that, therefore, his days will be limited to 120 years. Moreover, what has changed is that man has become wicked, and so God is grieved (Gen 6:5-6). This seems fairly plainly to suggest that, because man is wicked, he should not live so long and his years should be cut short. Note the author is being very explicit here in passing about something readers often observe to themselves, namely, that the lifespans of the postdiluvian patriarchs became gradually shorter; while Noah, who was born and lived most of his days before the Flood, lived to be 950, Shem lived to 600, Abraham to 180, and Moses to 120. So a broad gloss of the thought being conveyed is something like this: “My spirit, the spirit of God which I blew into Adam and which gives man life, will not always abide with and govern man—and thus he is mortal—and thus also he will not merely die, as all but Enoch have done, but he will live only 120 years because of his wickedness which is indeed “from his youth” (Gen 8:21).