Some angels have existed since the beginning of time, and are the channels through which divine plenty flows into the world.
There is, however, another kind of angel, those that are constantly being created.
This process of the creation of new angels takes place as a consequence of actions and phenomena that are performed and occur in all worlds, but especially in our world, the World of Action.
It is said that with every mitzvah, every good deed that he performs, a man creates an angel.
–Rabbi Adin Steinsaltz
From “World, Angels and Men” in The Strife of the Spirit by Rabbi Adin Steinsaltz