Damien didn't even expect he would kiss her. But his heart pushed him forward while his mind suddenly got shut down.
He not only knew about Valentina's feelings for him, but he also knew in his heart that he loved her way more than he thought.
But he had been suppressing them and keeping them bottled up because he made himself think that Valentina was too young mentally to understand love. And also, because she was a half-ethereal beast, he didn't know if she could love a human like him as her mate.
However, the true reason was that he was afraid if he was worthy enough to love and protect her. After losing all his loved ones in his previous life, he thought the more people he had close to him, the more chances he might lose them.
He was afraid, not knowing whether he could keep them all safe or let them down. Even after kissing her and deciding to confess his feelings to Valentina, he was still afraid.