Well..I'm combining 2 of my réponses here. Loong answer: Damon is always the one who gets her locket back, while Stefan is busy romancing her. Damon's her saviour always, he,unlike Stefan, doesn't ever lie to her & as he had said, it would be him who would keep her alive..& he would have, & she wouldn't have to be a vampire now. They are plus suitable than "Stelena", & now she is realising that (season 4) but is not yet ready to accept it. Again it will be Damon who can teach her how to be a good vampire. Damon knows what she NEEDS & is ready to give her that, even if it mean that she would stay human & eventually die. They had met first. Even though she chose Stefan, he's still there helping her, & whenever she has trouble with her "hunger" she contacts Damon, it was his blood she shared. His l’amour consumes her, & don't we all want that kinda love? Damon sorta grew on her & it is now difficult to shirk him off. How about the fact that he appears to be a better lover ( ;) ).. He's elder & hence wiser & seen plus of the world & is plus often than not right about everything. Listen to their phone calls..their banter is hilarious. Damon is over Katherine, & loves Elena truly..even when they had not "made out" ;) ..while Stefan seems to have some unresolved Katherine issues. He is a better person because of her..& as he challenges her she explores herself & gets to know herself. DELENA forever..while Stefan's & Elena's l’amour is simple & uncomplicated..Delena's l’amour is consuming..the kinda l’amour one would desire. Stelena is sweet & nice..but Delena is hot & sizzling & what True l’amour is like. toi fight..you hate..but toi absolutely l’amour & are ready to do anything. Damon, when he realised he screwed up the ritual thingy & that Jenna was gonna be turned..offered himself to be killed.. He gets a chance to be a better person 'cause of Elena, he's..DAMON is UNSELFISH for/because of her! Stefan's l’amour is fairy tale l’amour (though even the montrer is not reality), where he is the Prince Charming. But Damon makes her question herself & her life. Even though Stelena fans would disagree with me..but Damon & Elena's understanding is actually better..as they started off as friends..& that's the place to start..They got to know each other better before they felt anything for each other. Elena gets jealous when Damon was pretending to flirt with Rebecca..when was she jealous of Katherine going after Stefan..even when they were together?
I'm attaching a video I found on youtube..some of the reasons are kinda BLAH..but some are sound :D
Also the pic..as a proof that Delena dances are the Epic & Traditional dances..the way it is meant to be..as with them.
Here I rest my case. Any queries,,more proof..please feel free to ask :D
Also I know this answer would only go well with the Delena fans. I have nothing against toi Stelena fans. *Peace*