I think Dean actually made this tape when he was young and has been carrying it around with him for years. It’s super special to him because it has all his favorite Zeppelin songs and it’s the tape that has been played so much it’s nearly worn out. He and Sam have played that tape and sung along with Robert Plant on a thousand cross-country drives. And this tape probably got him through a whole lot of lonely drives after Sam went to Stanford. He knew that those songs were special to John and Mary and it’s become the soundtrack of Sam and Dean’s lives.
Under all of his tough guy bluster, Dean is a sentimental romantic. He wore the Samulet for years. He treats Baby like a living thing because it means family and home. He kept Castiel’s trenchcoat in his trunk for months after he died, even moving it from car to car. And his most treasured possession is a picture of his mother. Dean puts deep meaning into the few items he holds dear.
So the fact that he wanted to gift this (and wouldn’t take it back) to Cas means so much. It means that Cas is family and that he wants to share with Cas something that is an integral part of who he is. For a man whose entire earthly possessions can fit in a car trunk and a duffel bag, to give something that you made yourself and kept with you for maybe decades to a friend is a truly special gift.
Also, we need coda fic about exactly when Dean gave Cas a mixed tape and how exactly that went. As we know, Dean isn’t exactly *smooth* when it comes to flirting with dudes, and how else is giving someone a fucking mixed tape supposed to be viewed other than flirting?
You know Dean was an awkward little spazz about it.
(hence why Cas probably thought he needed to return it.)
Back then, you'd have to put a tape recorder up to a speaker or stereo to make a mixtape. Sooo that means Dean took time out to make Cass a mixtape... i don't even think he has made one for Mary or Sammy
All that time and effort he put into that tape. Picking the perfect version of each song. Maybe he wrote a few of his favorite lyrics on the liner card. Maybe he recorded voice intros to each song explaining why he picked it.