Looking back on it now, in a way Usui has difficulty in getting close to someone because of his little to no friends. The way he doesn't always talk what's on his mind, especially when he's worried is such a boyish thing to do for his age, despite mostly being mature. It's a treat to see him lose composure. There's also the complexity of Misaki being called a girl that she subconsciously associate as an insult from men instead of being a reason for target that Usui was trying to imply.