Disclaimer: Unfortunately I don't own anything apart from the plot.
Desmond had his limits. And he was close to reaching them. He was getting sick of the uncertainty that her glances were causing him. He wanted to be sure that the message she was trying to send him were the same ones he was receiving. Patience had never been one of his strong points but he knew better than to stroll over there and demand to know how she felt about him, especially not when Charlie was around. When they argued it took him all his restraint not to go over there and comfort her.
Yes, Desmond had his limits and Claire was testing them.