Commenting, when done well, coaches revision.
Revision leads our students to understanding more deeply what it is they want to say.
Revision leads our students to a better understanding of what readers need and want. Revision is a higher order thinking and will often influence students abilities to think more critically. Look for the promise of a draft rather than its mistakes. See yourself as responding rather than correcting. Rather than commenting copiously on everything, limit your comments to two or three things the writer should work on that would immediately strengthen the draft perhaps in dramatic ways.