I think it's easier giving with the right arm than with the left. I don't know if it's just because I'm right handed or if it's the same with everyone. Other than the handedness argument, the only thing I can think of for why this is so is because your heart is on the left side of your body, so when they suck your blood out of you from your left arm it's sucking it right out of your heart. I always feel dizzier and weaker when I give with my left arm. Should I just give with my right arm all the time?

They always take your blood pressure from your left arm. Because it's on the side of your body with the heart and they get more accurate readings.