As a cut off angel, it's hard to blend in with the normal crowd.
However, Castiel is doing quite fine, outliving everyone else as a simple cashier at the local Gas 'n Sip. Sure, he may never understand humans, but it's a privilege to watch them go about their simple lives, while never progressing in his own.
He just has no desire to; seeing that he hasn't found anyone worth falling - literally and metaphorically - for.
Just a fic inspired by a fanvideo. I don't own SPN, of course. The video nor cover art I didn't make, but the plot is entirely mine, only lightly guided by the destiel trailer "The Fall."