As soon as Eric's fangs pierce the skin and that first taste of Thranduil's blood rushes into his mouth, he is filled with a similar sense of ecstasy. He must keep himself in control, he knows - it wouldn't do to drink too much and cause the other harm while Thranduil is giving himself in an exercise of trust as he is. It would be so easy to just let go and lose himself in this moment, in the warmth of the flavor of his blood, like liquid sunshine and contentedness. He must be strong. But for now he will drink and savor this moment and the euphoria it fills him with. Thranduil's blood really is incredible.

He doesn't need to feed that much but it's the principle of the matter, and the fact that Thranduil is supposed to be learning to trust him, that keeps Eric latched on a little longer than he needs to be.