OK, how about this: Dean is more forgiving of Cas than Sam because Dean has much, much, much, much, much, much, much, much, much, much, much, much, higher expectations of Sam. Sam is his idol whom he relies on to be his touchstone of goodness so Dean feels utterly lost and betrayed when Sam topples from the pedestal Dean expects him to sit on, whereas Cas is only an angel and a dick by nature. Yeah, that must be it.