"Moshe said to them, 'I am one hundred and twenty years old today" (Devarim 31:2). Meaning, "'Today' is the exact day that my years have become complete. On this very day (the seventh of Adar) I was born, and on this day I will die." In light of this, the verse says, "HaShem knows the days of the wholehearted" (Tehillim 37:18). Just as they are wholehearted, so too, their years are whole. HaShem grants full years from the day of their birth to the day of their death. Just as a perfect circle is perfectly round, so too, the tzadikim have no "rough edges." All of their deeds and everything they have, even their days and years, form a perfect circle of goodness.
The perfection of a tzadik is his wholehearted love for HaShem and all of His creations. Therefore, the tzadik relates to every being with love, peace, and joy. Just as his love is whole, so too, HaShem blesses him with whole years.
Envision the joyous countenance of Moshe Rabenu. He has no contention with any man. All the days of his life are filled with love, peace, and joy. He loves HaShem and all of His creations. HaShem grants him a full one hundred and twenty years, in accordance with his pure and unadulterated wholeheartedness.
(Based on the commentary of Rashi and Da'as Torah of Rav Yerucham)
