Ever since you've met Diluc, each of your greetings started with a hug attempt; opening your arms, only for Diluc to ignore your gestures. This went on for several months until finally you've simply given up and greeted him normally. He recently noticed it, and something about it bothered him, like he finally understood that he appreciates your gesture. He craves your affection, and he wants to see you give it to him. So... He opened his arms for you to take.