Because at all times, the effect of force is mutual.
The Demon King is not always dominating Erin's decisions; occasionally, the latter takes the initiative in making unexpected choices together.
The other meaning of mutual benefit is just that, and Luo Yan is certainly aware of this. He is a master at pretending to be ignorant, but not truly ignorant.
The two joined hands gradually grew together; speaking of one side's influence on the other, it is very much like a tyrant contemplating themselves.
It must be said that although tyrants in the feudal era always refuse to admit it, they are indeed influenced by the peasants they govern, but that's only because they refuse to look in the mirror.
The coldest physical laws are never swayed by anyone's will; naturally, they also align with the iron law that "the effect of force is mutual."
