Perhaps you are one of the squeamish who faint at the sight of blood or in situations that may lead to blood (surgery, the insertion of needles, etc.). Your collapse is brought on by a sudden drop in blood pressure (among other things) that causes you to go weak at the knees and your head to get nauseatingly fuzzy.
There is a plausible evolutionary reason for syncope (SING-kə-pee), or fainting, at the sight of blood: the drop in blood pressure is a physiological response adapted to minimize the loss of one’s own blood. With a lowered blood pressure, the volume of blood that will be lost from an open wound is significantly diminished, perhaps sometimes being the difference between life and death.
Though selected for individuals, the sight of someone else’s blood still activates this adapted response. Apparently, evolution has an itchy trigger finger.