On the first week of my blood draws I would look away when I was pricked with the needle. Sometimes I would feel nothing, others I would feel pain, sometimes my arm would hurt afterwards and sometimes it didn't. I became curious: what makes a prick painful or painless?
So I decided on looking at what the nursing assistants are doing when they penetrate your skin with the needle. It makes you squirm at first, but it only takes three or four pricks to grow somewhat comfortable with it. I write somewhat because indifference to the stab requires a kind of concentration akin to meditation so that you don't flinch at the needle goes through your skin.
The reason why some stabs hurt has to do with the speed and steadiness of hand of that pokes you. Some hands are shaky and dubitative, others are steady and assertive with the poke. Your own state influences the state of the assistant, so people who are nervous about being pricked make inexperienced assistants even more anxious.
I am fortunate to have "nice veins" in nursing parlance. Unfortunately it is not an aesthetic statement but a practical one: my veins are bulging and visible so they are easy to prick. Some people have to be stabbed several times before the assistant finds the vein. Some people require experienced assistants to find them.
I have observed that--as I steady my mind and my arm through my breath--the painful pricks have stopped almost entirely. I can't write this with certainty, but I infer that, if nervousness is contagious, then focus must be contagious too.
The arm hurting after the blood draw has to do with the care of the assistant not moving the needle around as they go about their work. The needle is inside a small catheter which houses a small removable tube, and the tube needs to be inserted and removed while the needle is still in your arm. If the needle wiggles around during these operations it will cause pain later.
What I find most interesting in this situation is the illusion that you don't influence the outcome. It is oft repeated: don't stress over situations that you can't control. I would change that to: relax attentively over situations that you can't control, because your state is contagious, and a calm passenger can help prevent a driver from crashing.