However, a Heavenly angel disagrees, for once an angel encountered Rav Ketina wearing a linen garment without a blue thread in tzitzit - following the opinion that this is not needed. Angel said, "Ketina, Ketina, since you wear linen in the summer and a woolen cloak with rounded corners in the winter, what will become of the mitzvah to have a tzitzit with blue threads?" Rav Ketina said to him, "Do you then punish for failure to fulfill a positive commandment?" The angel said, "At the time of anger, yes."
From here we see that the angel considers tzitzit to be an obligation of a person, for why would he punish a person for the obligation of a garment?! - Not necessarily! He just told Rav Ketina that he was using loopholes to get out of the obligation.
Art: Pere Serra - Angel Playing A Harp