Mel's post on rethinking grading just came back to me. I hardly remember commenting (this past week is pretty blurry), but I think this is why I do get down when I'm grading:
I think that I feel bad when I'm grading. This is irrational because I do not take credit for a wonderfully written paper (ah, I helped that student so much), but I do tend to spiral downward when I see the 50th instance of its instead of it's or one form of there used to cover all the options. I feel like I've missed the opportunity to teach them something once again.
foolish of me, I know. I don't delude myself about being so powerful in any other endeavor. But here, one by one, they manage to convince me that I could have done better.