The Seven Deadly Sins stand equally amazed with how incredible is Lucien's potential to be the host of all of them. Even knowing him so well, Lust also doesn't quite understand how he can be that way.
It's evident that part of that is possible because his relationship with all kinds of emotions is very intense. But well, his sisters are like that too, mainly due to the influence of Bloody Rose, which makes them feel everything amplified.
Yet, Lucien seems to have something else, something special, that makes his relationship with the Sins so smooth and natural. Also, his relationship with his sisters benefits both of them in surprising ways.
The best examples of that are how thanks to him and only together with him, Envy and Amelia have such a powerful relationship with jealous and envious feelings. Being envious and being envied never had such an incredible magnitude before.