Yeah, I think there’s a very large love between Anna and Castiel. I was on stage with Misha Collins at a convention, and we were talking, and we both kind of feel like even though angels don’t really have sex in the way humans do, there’s something more there than just being co-workers…
Castiel/Anna hit its peak in Season 4, which had a subplot concerning their long history and complex relationship. Castiel/Anna fanworks experienced a steep drop sometime in Season 5 and Season 6, most likely due to the events of 5.13 The Song Remains the Same, and has never fully recovered.
Sometimes this pairing is considered incest - also called angelcest - depending on whether all angels are seen as siblings or not.