My haggard "I'm tired, overworked, and just want to be quiet right now" look has, I've learned recently, been misinterpreted as a "get the heck away from me, I'm angry" look. I hadn't the slightest idea.
Taking things personally, whether it be comments on presentation style, experimental design, theoretical derivation, or some other area that we all--hopefully--negotiate and learn, is not an effective way to play the game.
People don't leave because they can't cut it; they leave because they find some other place that welcomes them, lets them express themselves, and rewards them according to their merits.
Unfortunately many of us get tripped up in the hurdles, because we lack preparation, confidence, test-taking ability, motivation ... not necessarily because we don't have potential as scientists.
What you don't always think about when starting graduate school is that sometimes the choice to go to graduate school--especially when you haven't already met that special someone--can delay or impede your social progress, even as it advances your career.