What if Cas never listened to the mix tape, but instead just kept it in his pocket at all times because Dean gave it to him and that makes it precious.
Aww nonny why?? Saying that Cas never got the chance to listen to it before he died makes it SO much worse!
I like to think that Cas was listening to it in his truck when he was hunting Lucifer with Crowley to block out Crowley’s constant chatter. With a small smile on his face whilst he thought about how the lyrics reminded him of Dean… and Crowley would be like “Urgh… I never thought you would be one to listen to this rubbish. I guess Squirrel really did rub off on you…” and Cas just rolls his eyes and ignores Crowley because he’s too busy listening to The Rain Song.
Whenever he leaves the truck he takes the mixtape out of the player and puts it back in his inner coat pocket though, because he likes to keep it close to his heart.