Obviously drinking blood is deemed very wrong in almost every society, and it isn't good. But it gets to a point where it's required.

You haven't been taking what you needed too, and Barry has obviously noticed. You had certain pills that you started taking to mimic the nutrients that blood gives, but he knows the bottle is still half empty- and it's been like that for a while. The supply of animal blood that you usually buy has just been sitting in the freezer. And everytime he wears a shirt that shows any amount of skin you remind him that it's cold out and he should cover up.

He had a guess on why you stopped taking it; being a vampire surrounded by humans in such a culture where things like that seemed disgusting- you were bound to feel out of place. Feel bad for needing blood. But he didn't understand why all the sudden bow you were stopping.

The effects, of lack thereof, were obvious. You had that paler complexion, that certain look in your eye while you carry yourself. You needed blood, and he needed to convince you it was okay. It was his duty to keep you healthy, and he was prepared to make sacrifices.

"You need to eat." He puts on a soft yet brave voice as he rolls up his sleeves. Was he nervous? yes yes yes a thousand times yes! but you needed blood. And he was a whole body full of blood. "Please, I'm here if you need."