A non-shipper's view of the mixed tape
I’d like to begin by stating that you should feel free to ship - or NOT ship - whoever you please. This is simply the way I see things, and I’m not saying you have to see it the way I do.
Cas is Dean’s best friend, his family. An angelic best friend. Now, angels, ESPECIALLY Cas, are notoriously ignorant when it comes to pop culture - music, movies, books, etc.
Dean Winchester is a lover of rock music down to the deepest depths of his soul. It’s an important part of who he is. Cas, his best friend, is unaware of who these bands are and what the genera even is. Imagine when Dean finds out that Castiel doesn’t know who Led Zeppelin is.
Dean: *ranting about how great Led Zeppelin is*
Cas: An airship made of lead seems extremely misguided.
Dean: *thinking* He doesn’t know who they are?! Oh HELL NO. I’m fixing this now.
Dean would be horrified if his best friend found music on his own and started listening to the likes of Taylor Swift or One Direction. I don’t see it as romantic, I see Dean giving Cas a mixed tape as an effort to develop Cas’s astoundingly non-existent music tastes in the likeness of his own.
I mean, hell, if my best friend was an angel that had never heard of or listened to my favorite band, you can BET I’d make them a playlist and shove it into their hands.