Giyu had always trouble getting close to others due to being aloof which in turn was mistaken for arrogance to the point of being viewed with apathy and sometimes even disdain. The fact he is a man of very few words also didn't sit right with many.

And due to being a loner with a false bad reputation he didn't even know about he was still surprised you took a liking to him. You were a pleasant distraction from his usual silent life, warmth he had needed.

The fact the two of you started working the same mini job during your time at university, even after work you lingered by his side, making an effort to have an actual conversation with him.

You wanted to keep it a secret, after all people wouldn't understand why you were dating him in the first place and he didn't mind until he figured out it all had started out as a bet you had made.

The warmth you had managed to place in his cold heart began to rot as the words of the secret arrangement kept repeating in his head. The thought of leaving you behind did stuck with him but at the same time he didn't want to be alone again. So for now he stayed, remaining suspicious of your actual intentions.