The Directory on Popular Piety and the Liturgy states: “Popular devotion to the holy angels, which is legitimate and good, can, however, also give rise to possible deviations . . . [such as the] practice of assigning names to the holy angels [which] should be discouraged, except in the cases of Gabriel, Raphael, and Michael whose names are contained in holy Scripture” (217).
Perhaps the practice is discouraged because guardian angels are creatures of God, with their own personal dignity and vastly superior nature and intellect. They guard us because it is the will of God that they do so, but we should not presume to know the name given them by God and should not presume to demand a name of them. If such a name is revealed without our initiative, then that would be a private revelation that should be vetted first for supernatural origin by a trusted confessor or spiritual director.