Responses

I think it is because he wants to keep him by his side always and keeping protecting the MC on his fragile phase, so Aki is enjoying this feeling. Once Yo becomes complete vampire he will gain more freedom and be stronger meaning that he will not be so fragile anymore and may not be dependent on him fully.
I see why he would be anxious about Yo finishing his transition, but he will learn that their bond will not break and become stronger ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄
I may be completely wrong here but this reasoning make me feel better ٩(๑❛ᴗ❛๑)۶
Why did he felt upset when he thought he was about to completely transform?