They were fighting on who’s the greatest dad/the better dad to you. (They did this to Charlie a few months ago)

Alastor: Your just like the child I wish I had!

Lucifer: Uh- What?

Alastor: Your just like a daughter I spawned!

Lucifer: Wait hold on now!-

Alastor: It’s a little funny cause you could almost call me… Dad!!~