Bucky's words catch Thanos off guard. It has been... a long time since he had a conversation. A real conversation with someone ostensibly an equal. He was surrounded by enemies and yes men and children. People who did not speak plainly to him, or spoke with obvious antagonism and need not be taken seriously.

The word 'ugly' has a particularly striking meaning to Thanos, shooting him backwards in time to Titan.

"Is ugliness so terrible?" he asked.