Y’all know what I love so much about the mixtape?
It is now canon that at some point, Dean Winchester sat down and recorded all of that music, thinking very carefully what to put on it for Cas. Then he went and gave it to Cas.
Let me stress this, at some point Dean Winchester gave Castiel a gift and he was probably all grumpy about it and trying not to make a big deal out of it, but he still gave Cas something for his truck, something to remember him by when he is on the road. That is such a sentimental (dare I say romantic) gesture.
This fact is now canon and it opens the door to so many more scene like this.
There is so much more to their relationship than we get to see.