The whole reason Cas is able to so easily put aside what Dean did was clearly laid out for us when he said, "…there’s a little monster in all of us."
It has nothing to do with his preference toward Dean. It has nothing to do with his love for Dean. This has nothing to do with shipping. He was able to do this with Sam at one time. He was able to do this with himself to an extent. Maybe them being good friends plays a small role, but the fact of the matter is, is that Cas knows all the fucked up shit he’s done (killed hundreds and thousands of his own kind and killed tons innocent people under the influence of the souls/leviathan; he killed Dean a thousand times while under Naomi’s control). He knows all the fucked up shit that Sam has done (under the influence of demon blood and ruby; under the influence of having no soul; ffs he drank demon blood out of a possessed woman while she screamed for help because the demon was asleep inside her because he thought it was all he could fucking do). He knows what it’s like to be desperate and pay for it.
Dean is under the influence of a force he has no idea how to control. He took on the Mark despite warnings, but the consequences weren’t laid out for him other than a vague warning of “good luck you’re gonna need it.” Taking on the Mark was the act of a desperate man (kind of like Sam with the demon blood and Cas with the souls of purgatory). They’ve all done some really messed up shit that to anyone that doesn’t know them might be unforgivable. But they are able to forgive each other because they know.
Claire, in an act of desperation, found out that even she has a “bit of monster in her.” She found out first hand what it felt like to be filled with rage and desperation to the point of being willing to kill “innocent” people because of things that have happened to her outside of her control (i say innocent in quotes because Dean is not exactly an innocent man but he can’t really be blamed for the carnage that was the end of the midseason finale). We were shown through Claire that even without a supernatural influence, people could be capable of really monstrous things. But we were also shown that people can also be capable of holding their monsters back.
Cas’ forgiveness doesn’t come from a place of naivety. It’s not supposed to be funny. It’s not supposed to be like “haha look at what his friend did and he loves him so much he can’t even put him down.” His ability to forgive and hope and seek out a way to help his friend rather than kill him comes from experience and empathy.
That forgiveness, of others and oneself, and the realization that only you can chain up your own monsters is the point that’s going to push us through the rest of the season.