Lucifer has experienced the most gruelling pain when he and his brothers got casted out of heaven but none were as painful as you being Diavolo's attendant instead of his.

Many years ago Lucifer had declined on you being his and his brothers' attendant. Of course you showed no hard feelings and still was friends with his siblings. Lucifer began to regret his actions when got to know you and lovely you are. So it hurts him to no end when sees you being Diavolo's attendant instead