So much of Arthur's personality made sense to Maria after he told her about his mother and how lonely his childhood was.
All of his life, he had been tossed to the side, kept away, and belittled by everyone-- family and servants alike. Even with being a prince and next in line for the throne, he was never respected or taken care of in the way that his title would have demanded.
Not only that, but he had lost the one person who truly seemed to have loved him.
It made sense that he took on this persona of an overly-confident, stoic prince with people who did not know him outside of his home; he could not come across as weak, and he did what he could to survive and take back his power as a person worthy of respect.
He hid his emotions behind a blank expression, and he hid his true self from everyone in order to protect himself so that no one would hurt him again.
For the last nearly twenty years, Arthur had to make it on his own because he had no one once his mother was gone.
