People have a lot of reasons they get into science as a career, but no one ever seem to go into it for the rejection. This is notable because if there one constant in a science career, it's that you're going to get a lot of rejections. And we only have ourselves to blame.
We're taught to be critical of everything all the time. Don't accept your results at face value. Critique pushes science forward (especially when it's of that other lab!) and keeps us thinking. But everyone has spent time in that journal club or lab meeting where it seems like the sole purpose is to tear anything to shreds. "Sure, you discovered the Higgs Boson, but you announced it in Comic Sans so is your work really any good?"
It's something I try to remember as a reviewer. I think my natural tendency is to focus on the things I don't like in a grant or paper. It's as though not being able to criticize something in a proposal is basically mailing in my review.
And I'm not alone, I've gotten my fair share of Excellent or Very Good rankings with a review including about a 4:1 hated it/loved it ratio. Critique is what we're good at. But I'm making a concerted effort this month to dedicate roughly the same amount of text to both the "Strengths" and "Weaknesses" sections of my reviews. Obviously it's not going to work out that way every time, but I'm reading a LOT of really good science. It's worth recognizing that in writing.