A common misinterpretation is that this verse establishes a universal cap of 120 years on all human lifespans from that point forward, overlooking biblical accounts of post-Flood patriarchs like Shem who lived longer; others mistakenly apply it as a promise of shortened modern longevity due to sin rather than a specific pre-Flood deadline for the corrupt generation facing the deluge, or divorce it from its context of divine patience exhausting before cataclysmic justice.
Genesis 6:3
King James Version (1769)
3 And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
God limits human lifespan to 120 years because humanity has become dominated by fleshly corruption, and His Spirit will not strive with them indefinitely, foreshadowing impending judgment via the Flood while allowing a grace period for possible repentance amid growing wickedness on earth.