Every mitzvah that a man does is not only an act of transformation in the material world.
It is also a spiritual act, sacred in itself.
And this aspect of concentrated spirituality and holiness in the mitzvah is the chief component of that which becomes an angel.
In other words, the emotion, the intention, the essential holiness of the act combine to become the essence of the mitzvah as an existence in itself, as something that has objective reality.
And this separate existence of the mitzvah, by being unique and holy, creates the angel, a new spiritual reality.
–Rabbi Adin Steinsaltz
From The Thirteen Petalled Rose by Rabbi Adin Steinsaltz