Sforno on Genesis 6:3/ספורנו על בראשית ו׳:ג׳

1לא ידון רוחי באדם לעולם בשגם הוא בשר, it is intolerable that there will continue the argument, strife before Me allowing (the angels?) to claim that although seeing that man has been endowed with divine qualities he must be punished, whereas his close ties to physical earth must be considered as excuses for his conduct, entitling him to continued mercy, and indulgence by Me.

2והיו ימיו, he will be given a certain length of time within which to repent his sins;

3מאה ועשרים שנה, during which time Noach would have time to construct the ark, and by doing so rebuking and warning people of what is in store for them. (compare Sanhedrin 108)