The trouble with drinking and sitting meant someone couldn't tell the true extent of how much they had to drink until they got up once again.
Leonor was experiencing that as she continued to drink and thought she didn't feel it, so she would continue to have more.
Desmond, with more experience around drinking and a bit more control, started turning away the drinks that Louis offered. He felt he should take the reins of the situation if they were to make it out of there in one piece.
Even though it was getting dark and the lights were focused on the town square, he didn't want Leonor to slip up in front of people she wanted to give a good impression to.
Despite his rushing thoughts, he knew he needed to be the one to have control of the situation.