Genesis 6:3
וַיֹּאמֶר יְהוָה לֹא־יָדוֹן רוּחִי בָאָדָם לְעֹלָם בְּשַׁגַּם הוּא בָשָׂר וְהָיוּ יָמָיו מֵאָה וְעֶשְׂרִים שָׁנָה
Re: the clause “my spirit shall not dwell with man forever.”
The conceptual background for this wording is Genesis 2, where God breathed life into the flesh of man. The phrase “my spirit shall not dwell with humanity forever” is a way of saying “the life span of humanity is going to be shortened” that recalls the context of Genesis 2 and reminds the reader that the life of humanity is completely dependent upon God. Humanity is merely flesh without God.
Brian W. Davidson 