Damon was your friend, you were dating Stefan, he was jealous, he wanted you. He loved you.

In 1864 Damon was in love with your doppleganger, she was playing with him and Stefan, she was the one who transformed them. Damon spent the last 145 years looking for her, Only to find out she wasn't there and that she just didn't come to look for him, she came back, thinking it was you he kissed her.

"Why were you surprised that I'd kiss you?" He asked, apparently hurt.