"Your eyes, they shine so bright
I wanna save that light
I can’t escape this now
Unless you show me how” (x)

~~Season 10 Scenarios~~

After Sam and Cas have tried anything and everything they could think of to save Dean from his demonhood, but had to realize that nothing works, because the key to curing Dean is for him to start embracing deeper emotions again (which is something Dean just isn’t open to do) they are close to giving up. What they need is a strong emotional reaction from Dean, that would make way for him to come back to his true self, but neither Sam, nor Cas are able to evoke that kind of feeling in him. In their desperation the two go to extremes and find a way to bring John Winchester back for a short period of time hoping he might be able to get the old Dean back. When Dean sees John though he merely laughs at Sam and Cas telling them, that if this is the best they got, they should better stop, because his father doesn’t mean anything to him. Hell, that he is the one to blame for half the things he struggles with. And Dean just shoots them an evil sort of arrogant smile and chuckles to himself while John dissolves and Mary Winchester appears in his place. And that, seeing his mother and her looking at him asking him what happened to him, to her little angel, stops him dead in his tracks and serves as a catalyst for the true Dean shining through and feeling that strong emotion, Sam and Cas needed Dean to feel again in order to cure him: Love.