Stage One: "The Prince and the Servant"
There were typical stages that a normal and healthy relationship went through. First there was was the infatuation phase. That was the point of a relationship where everything was perfect. The second stage was the imperfect phase, which was the point where people started to fight and find faults with each other. The third was the negotiation phase. At that phase invested couples tried to change one an other. The last stage was the commitment phase, where the couple came out stronger and worked out their differences. Merlin and Arthur did not start out with a normal relationship.
Stage one for Merlin and Arthur was the prince and the servant stage. It didn't last very long at all, since Arthur didn't treat Merlin like he treated most servants, and Merlin was never as respectful as they were. It only lasted a few days in fact. It equated more to the getting to know each other stage. Arthur and Merlin were mainly stuck in this stage when they were around others. Arthur would act as if Merlin was nothing more than just another servant to him, and Merlin would in turn try and treat Arthur like a prince. Of course Merlin was never very good at that. He did try at first.
Merlin's first day as Arthur's manservant was rather pivotal in their relationship. Normally it would set the tone for the whole relationship. Of course that was with normal people, not Merlin and Arthur. In fact the first days of Merlin's servitude to Arthur only served to make the prince more curious as to what lay underneath that sarcastic, pasty exterior. Arthur had never had complaints with the castle servants before Merlin came along. He was content with the spineless boot lickers. They did exactly as told, never argued, and basically let him walk all over them. Merlin, on the other hand, had no issue with arguing with Arthur, or even insulting the prince.
It wasn't until later that Arthur realized that Merlin actually made him a better person, and a better prince. However, he couldn't say he did the same with Merlin's skill as a servant. At least Arthur would never admit to it. He would always insist that Merlin was the most useless and incompetent servant ever, even if over time Merlin had proved to be the most loyal and kind person Arthur had ever known.