Disclaimer: I don't own Merlin.
Merlin's task had never been easy. Destiny had given him the job of turning an arrogant prat into the greatest king the world had ever known. Each day was spent molding the prince into a man the people of Camelot could respect, something that was not easy at all.
When he had first met the prince, he had tried to show the prince that everyone's life mattered, no matter what their station in life was. It took time, but Merlin was eventually able to convince the prince that no one's life was worthless. The prince began putting the needs of others before him.
Merlin tried to give his prince advice. When he could, Merlin would give Arthur his opinion, even if it was not asked for. Even if it did not happen all the time, Arthur would consider some of the things he had said. Eventually, he recognized that what Merlin said often contained valuable wisdom.
He tried to show the prince the true meanings of friendship and loyalty. He wanted to show Arthur that true loyalty meant showing constant support for someone. He wanted to show him that true friendship meant that people were always there for each other, were always willing to listen to one another, and were always willing to be honest with each other.
The process had been a slow one. It had taken time to mold Arthur into a new person. Eventually, the lessons Merlin had given Arthur had taken root in the prince. Arthur went from being an arrogant, egotistical, prat to a man that the people of Camelot could be proud. There were many times throughout the process that had made the task seem impossible. He had wanted to give up on so many occasions, but he chose to finish what he had started. He chose to continue not because his destiny required it, but because he believed in the man that Arthur was capable of being.
This was inspired by the quote "Constant dripping hollows out a stone" by Lucretius.