As they talked about how Sun felt like she didn't belong in their kingdom - how she felt being different from her brothers like she didn't shine enough and that she often spent her days wandering alone - Moon was surprised in horror that he could not help himself and let out a small chuckle instead.
Sun thought he was mocking her and before Moon could give an explanation, she stood up ready to walk away and because of panic, Moon suddenly reached out for her hand; and for the first time, they had a physical contact.