Getting hit on the head. I can stub my toe without any flash of emotion of any kind, but a little bump on the noggin or, to some extent, my upper back, shoulders, and neck, makes me want to kick a puppy. A large part of this is that most of the times I get hit there occur when I am doing certain juggling tricks, so the pain is accompanied with frustration all in a tight little package. And, as the specific areas indicate, another factor is my inability to see and articulate those parts of my body; so the pain feels instinctively like an external attack rather than the result of my deliberate actions.