Lady Serena was not pleased with the fact that this guy was an irritant. She wanted to dominate him and she did so using her pee. She lied to him that she had the hots for him but then when he fell for the lie, she turned around and she made him a pee slave. He had to lick her pee as well as drink it and she made sure he did it like she wanted.
Lady Serena wanted to show dominance over her slave and she used her pee to do it. She did not want her slave to take her for granted or to do anything that would make her get annoyed or angry. And she did not want to do it with pain. So she resolved to do it with humiliation, and on that, she used her pee to do it. He had to drink it.