sometimes adverse comments make you bitter, but rarely better. I'd always try to find something positive to say when grading student papers. In art, it's different. You're always going to be held to a higher standard, so even if you don't agree, grit your teeth, carry on and try to get better. Some criticism can be like encountering bumps along the path on the long road upwards, but remember, the bumps are what you climb on :)