Once long ago one of Uther's knights gave Arthur the runt from a litter one of the royal hunting dogs had. He named him Boris and Arthur loved him and took care of him. Boris slept in Arthur's bed in his nursery and Arthur gave him the scraps from his meals. Boris doted on Arthur, and Arthur grew more attached each day.
Until one day Uther bothered to pay attention to his son long enough to notice the young dog and Arthur's love for him. Uther had the dog taken out and killed while Arthur's nurse held him as he screamed. Uther pulled him from his nurse's arms and informed him that loving a dog was a weakness, and Princes did not show weakness. Arthur was five.
When Arthur was eight he made friends with a boy from the kitchens. His name was Robert and he was nine and the son of one of the cook's helpers. They snuck into empty rooms and ate stolen sweat-meats and played at being knights and spun in circles until they were dizzy and fell to the floor giggling.
Until one day a servant saw them together. And then Robert and his family were gone from Camelot and Arthur never saw him again. And Uther informed him that making friends with servants was a weakness, and Princes did not show weakness. Arthur didn't try to make friends again.
When Arthur was sixteen he lost his first tournament. He was defeated by a much older and more experienced knight who had broken Arthur's sword arm badly. While Gaius set and splinted his arm Uther scowled down at Arthur.
Princes were warriors. Princes were champions. Losing a tournament was a weakness, and Princes did not show weakness. Arthur didn't lose a tournament again.
When Arthur was eighteen he had his first lover. Lady Alice was the second daughter of one of Uther's lords. Lady Alice was a year younger than Arthur. They were madly in love as only the young can be with their first lovers. They made each other promises and swore to be together always.
Until Uther learned about it and then Lady Alice's father was given a large sum of gold and Lady Alice was sent off to marry a lord in a far away land. Arthur didn't get to say goodbye. Uther informed him that he would marry for the good of Camelot and for no other reason. Princes did not fall in love, love is a weakness and Princes did not show weakness. Arthur didn't take a lover for more than one night again.
When Arthur was nineteen he met Merlin. And everything changed. Arthur became attached, and he became friends, and he loved. But Arthur had learned his lessons well. He called Merlin an idiot or useless, and knocked him around, he shouted at him, and worked him to exhaustion. And if at times he found himself longing to show Merlin his feelings, to kiss him and love him freely, he could find comfort in the fact that no one knew, no one saw.
And so Merlin wasn't killed and Merlin wasn't sent away. And if sometimes Arthur's longing for Merlin was so great that it caused him pain, he would punch Merlin's shoulder and smirk and call him an idiot. But Merlin was safe, and Arthur didn't show his weakness for him.
Because Princes did not show weakness.