When a boy becomes a bar mitzvah, he becomes a metzuveh ve’oseh—one who is commanded and fulfills. The Ohr Gedalyahu explains that this is based on the gemara that גדול מצווה ועושה ממי שאינו מצווה ועושה, one who performs a mitzvah having been commanded to do so is greater than one who performs a mitzvah without having been commanded to do so.
According to this, a bar mitzvah boy is now a gadol—not merely because of age, but because he now enters the elevated state of gadol ha’metzuveh ve’oseh.
One who is metzuveh ve’oseh has the power of the very One who commanded him. This is reflected in the bracha we say before performing a mitzvah אשר קדשנו במצותיו וצונו, Who sanctified us with His commandments and has commanded us, meaning that through fulfilling the mitzvos, which is the will of Hashem, the kedusha of Hashem rests upon him and that’s what gives him the power to fulfil the mitzvos properly.
Likutim, Bar Mitzvah. Interestingly, when a boy becomes bar mitzvah, a neshama descends to him from heaven (see Sefas Emes, Pesach, 5645, s.v. B’inyan Shem Shabbos Hagadol).
Kidushin 31a.
