One of his brothers once said that Castiel's problem was that he had too much heart. He cares so much that he will willing throw himself in the line of fire so that others can survive. The proof is in the fact that time and time again he has in fact died, only to be brought back to continue his mission to protect and fight against those that threaten the things he cares about.
However, his knowledge is extremely analytical and despite being on Earth a number of years, there are still things like social cues and innuendo that escape the angel's knowledge. Castiel does pick up things quickly, for instance, he has recently learned emoticons and thus every text is filled with them.
His bluntness, however, does have it's good points, in that he doesn't tend to beat around the bush when delivering a message. And he tends to call others out when they don't get to the point quick enough.
He also takes a lot of outside criticism to heart and often plays it over in his mind so it warps into his opinion of himself. It is a dangerous way of thinking but he just needs reminding every so often that he is good just the way he is.
There is also the fact that his good intentions can sometimes lead to dangerous consequences for himself, including death. Unfortunately for his own well-being, Castiel doesn't care. He would die a thousand times and give up his grace again if it meant keeping those he cares about safe. But despite his intentions being well-meaning, he forgets the fact that others care what happens to him.
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