Thorin heard Bilbo's words and it made him grin a little, because it was the very thing that he had wanted to hear. He knew that he wanted to do that again and again. For some reason, he found himself drawn to Bilbo in a way that he hadn't ever been drawn to another person before. Knowing what awaited him back home, he decided to just go with this good feeling and allow himself to be happy for the first time in a long time.
Bilbo kissed him then and he wasn't worried about anything else; after only a second to process what was happening, Thorin pulled Bilbo closer and deepened the kiss. Something about it just felt right and he knew that he wanted it to happen again and again.
Finally, he leaned back from the kiss; he thought that it was a good idea to allow the pair of them to come up for air. "Well, that was the best surprise of the day" he said with a bit of a grin on his face.