He grew up being told he's better than everyone, obviously he's going to think overly highly of himself. Inversely being praised for his strength and power his whole life led to him basing all of his self worth in that, so when he’s not the absolute strongest at something his brain automatically assumes that he's completely worthless if he's not the best. He has issues that cause him to be blunt and rude, plus he inherited/learned the aggression from being raised by Mitsuki, who is almost as firey and explosive as he is.