My personal opinion is that Cas doesn't learn because he always gets pretty close to insta-forgiveness after he screws up. He kills 50% of all angels, within 3 seasons the angels are announcing they need an leader and CAS is the ideal candidate. He breaks Sam's wall. As soon as he's back Sam forgives him. He takes Jimmy away from his family, then when Jimmy is free extorts Jimmy into taking him on again by saying he'll use Jimmy's daughter instead then gets Jimmy killed? Claire is mad at him for like 5.2 seconds then decides that since he looks like her Dad and since she's been told her Dad died a hero that what Cas did was okey dokey. Release Lucifer from the Cage a second time? Totally ignored because he got trapped in his own mind.

People were still banging on Sam for releasing Lucifer through what, season 11? Cas looks all woobie and says he feels bad and THAT is apparently enough to get him a get out of jail free card. Why should he learn that what he does is always wrong? Other people are the ones who bear the consequences of his decisions, Cas, not so much.

Sorry, it's one of my great problems with the character and story line for Cas.