1. They have chemistry
2. They 'road' is long and hard but it makes it better becouse we can see how their feelings are growing
3. The way Damon loves Elena, and his selfless is wonderful
4. He want to be better person for her and becouse he isn't a human he feels depressed
5. He is (after Rose) only one who understand him, and cares about him.
6. Thair dance was magic.
7. The way he kissed Katherine when she was pretending to be Elena was so gentle.
8. Elena is going to have more fun with Damon
9. Becouse of book they could make Damon human and they could 'live happily ever after' (it sounds a little strange i know)
10. He deserves her.
11. She can't hide her feeligs forever
12. Even Stefan know that Delena is happening.
But in my opinion if you love Delena you should know why :)