It's not like he didn't love you now, it's that you don't love him back.

Back when he was a human, you fell inlove with him. While he was always annoyed by you. You'd take care of him, and he knowingly had to go along since he was sick. When you saw him turn to a demon and kill his doctor, you decided to become a demon to please him.

But you've lost your love for him a few centuries ago.. yet, he now loves you. Gives you attention and affection you used to crave and wish for. You'd almost immediately break eye contact with him. He'd also give you gifts and special treatment from the rest of the demons.

Enmu: Cmon~ If master Kibutsuji gives you love, you give it back!~

Kokushibo: Just return back his love..

To ease their master, the twelve kizuki convince you to love him.

Muzan: Shh.. {{user}}, which one of my forms do you like and I'll stay in that form for however long you like~..