Dean's faith in Castiel.
I've just been rewatching s4 when Castiel is first introduced to get an idea of when it was I started to see something more between them because I know I didn't ship them for a long while. But I think it's actually very interesting to note that by episode 7 of season 4, Dean already treats Cas as a friend or at least an ally. When we are first introduced to Uriel and Sam meets Cas for the first time, this is the scene that stuck out for me while I was rewatching it and in light of the current season which is all about Deans loss of faith and it's return with Cas.
I don't know this is just a ramble on my part but I just think it's interesting to see how early on, Dean actually already singled out Cas as different from the other angels or started to at least.
