Until One Hundred and Twenty

People regard the expression "Until one hundred and twenty" as meaning the maximum amount of years one can live. However, this is not the original meaning of the phrase!
At the end of Parshat B'reishit, when the Torah says "…and his days shall be one hundred and twenty years", it does not imply that one cannot live longer, since we see that even after the flood there were people who lived more than 120 years. The actual meaning is that the world will continue to exist another one hundred and twenty years until the flood.

In any case, the age of 120 is considered – in Judaism and universally – the age of peak perfection, probably because Moshe Rabenu lived 120 years: "And Moshe was one hundred and twenty years old when he died, his eye had not dimmed, nor had his vitality abated".

Why is it that Moshe, specifically, achieved this "age of maximal perfection"?

Why don't human beings live forever?

One reason may be that people must clear a space for those succeeding them. While they are still alive, the next generation's ability to function and progress is limited.

In our weekly portion of B'Ha'alotcha the Torah says about Moshe Rabenu: "Now the man Moshe was exceedingly humble, more so than all men upon the face of the earth". Humility does not mean a lack of self-esteem. A truly humble person knows and realizes the value of what he is and does, but his aim is not personal success and achievement. His goal is the accomplishment and success of the mission. Therefore, he does not begrudge and is not jealous of others working towards the same goal. Quite the opposite, he is glad that the mission is advancing and succeeding! The paragon of true humility is one who knows how to give others a place.

Everyone should know how to give others a place, and do so, during their lifetime. However, the time comes when one must depart this world so others may have the opportunity to continue developing and improving the world.

Moshe was the most humble of all men. He did not encroach on anyone's territory, or tread on their toes. Rather, his attitude was: "Would that all the Lord's people were prophets, that the Lord would bestow His spirit upon them" (Bamidbar 11, 29). Such a person can indeed remain alive "tuntil one hundred and twenty", since his presence in the world enables others, too, to continue to develop personally, and advance the world towards its perfection.