Katherine smiled, mirroring his more alert stance, pleasantly surprised by his answer. He had told the other Katherine he needed time to think (because of her?) but he was happy to promise himself to her forever. She had never really let herself dare to believe, since returning to Mystic Falls, that Damon would ever promise himself to her like this again. Her smile widened as she took it all in. She had her Damon; had what she had always really wanted. She nodded at him slowly before leaning in to kiss him, slow and lingering - full of passion and love. The idea of eternity with one man was a scary thought now it was a reality - that was what he was asking for, wasn't it? Faithfulness? - but this was everything she had dreamed of and she couldn't lose that just because of her own fear of commitment.
"My Damon," she purred against his lips, opening her eyes to fix him with a loving gaze. "Now, please don't be sad any more, my Damon. You'll break my heart. I do so hate to see you sad." Subtext: she hated seeing him upset over some other Katherine. She would show him she was the better choice, that she would always make him happy. She couldn't help being annoyed at Katherine for upsetting her Damon. It wasn't like she was an angel. Even if she was from before she Salvatores, she had done plenty of less than honourable things in the past. She could hardly blame Damon for such a slight indiscretion. Especially when he was so cute and adorable and fluffy... Yes, she definitely liked this development. This Damon was too good to share.