Jon called "Snooky" his BFF, his partner, and his shrink; so it's likely a combination of things: stability, friendship, a shared vision, chemistry in the studio, just to name a few. Plus, I imagine the paycheck's not too bad either. Jon could probably outbid almost anybody who wanted to hire Shanks away from him, if it were to come to that. But I doubt it will. If @IleneToTheWrite can be trusted, one of them has wanted Shanks in the position he's in for quite some time. Jon clearly believes what they have now is working; and he's said numerous times that he doesnt like change. Barring something unforeseen happening, my guess is that Shanks is here for the duration.