“If you are profiting from being unhappy you will find ways and reasons to be unhappy.”
I believe the idea expressed in ‘appetite comes with eating’ is that your desire to indulge in an activity is correlated (caused even) with that activity. IE skydivers will likely have a larger appetite or desire for skydiving than a layman.
In a larger context I believe cjbgkagh is arguing that women can retain an advantageous position in a marriage by pretending or presenting unhappiness because exiting a marital contract would be an especially poor outcome her partner.
It's more of a general phenomena that I believe commonly applies to this specific case. Often there is an assumption that a demand can be satiated once met, but quite often instead the demands simply increase.
Sorry if this is a common expression.