I am severely phobic of needles. Blood draws cause my blood pressure to plummet, which can cause any number of other major problems with my overall health. Furthermore, it's very difficult to get a good vein in my arm or in the back of my hand. They almost all roll, and it takes at least two jabs by even the most experienced nurses to get a good vein. The experience is ridiculously traumatic.
If it weren't, I'd give blood freely. More pricks only make me more afraid of the needles though, so exposure therapy isn't the right path on this one for me personally. I've always felt bad that psychologically I just can't get there.