Simon didn't understand until the bad days.
He sees a Jayne who acts like he used to, maybe he still doesn't purchase whores, but he still drinks and gets into fights like he always did.
Then he sees a dress as they are out together looking for supplies, the sort of dress she would have worn and suddenly Jayne is wrapping him in a bear hug stronger than anything he's ever known, tears streaming down his face.
And Simon knows that everything Jayne does is all he can do to keep himself from breaking down.
Simon didn't understand until the bad days.