Being evaluated

I've gotten evaluated twice in the last two days. Yesterday I got my student evaluations from this last semester. They were average to good, with the comments not being unexpected and mostly about things that I have already decided to change for the next semester. There was one comment, though, that just eviscerated me.

I got comments back today on the chapter of my dissertation that I submitted a few weeks ago. On first read, they seemed horrible, but on second read it appears that about half of the chapter is okay, I just need to write a new other half of the chapter to go with it.

Why is it, that even when the bad is liberally mixed with good, it's the bad feedback that I remember and dwell on?