As Lucien strokes and kisses Amelia's belly and breasts, she can feel how much he desires her. She can feel the 'lust' he feels, and that is the same as she feels.
Becoming strong than Eve and Donna has always been a secondary goal for Amelia, while her heart's real desire was to be loved very much, especially by Lucien, who seemed the least improbable to do that.
Maybe her feelings didn't start out as genuine as Sophia's, and she was just jealous, but now, feeling how Lucien really loves her, she can't help but be extremely satisfied.
Meanwhile, Envy, who receives the same treatment as Amelia, feels very good and doesn't think much about it. Although she doesn't have the same connection that Lust has with pleasure, Envy has also always liked to feel good. In fact, getting what others want most has always the way she feels good.