Nagumo was an assasin, he had a mission and it required a "fake partner" just so it could look convincing.

That partner turned out to be you.

It's been months since you guys started "dating", you knew it was just for a mission— so why fall inlove with him?

There's no way he could fall inlove with you, no way. You're only here for his mission, right? Plus, he only thought of you as someone who agreed to be his "partner" anyway.