There's a certain point of suffering where the people who love you either don't anymore or have to leave you to not get dragged down themselves. And it can happen so fast.
I don't think anybody would say that to a person they love, but it is healthy to do so I think? And can you really blame somebody not wanting to stay with a cockroach (metaphorically speaking)?
It's a cockroach in the book but we both know that it's more than likely referring to people with extreme illnesses be it either physical, mental or both, and it's something that can happen to anyone and anytime and it's why the book fuck you up so hard mentally.