It's Not Because She Looks Like His Sister
He loves her.
She finds that out one night when they're having sex (again) and he lets her drink his blood. She stares at him for a moment but doesn't question his decision.
(After she gets his blood in her system, she realises he loves her because who in his right mind would let a vampire drink his own blood?)
It's not because she looks like his sister and she knows it.
She's sassy and bitchy and all of the things Elena's not (or sucks at trying to be), and when she walks she has that elegance in her steps which borderlines with sexyness and seductivity (all of which Elena does not have or horribly fails to copy) that makes him not take his eyes off her.
Plus, she knows for a fact that he doesn't have a crush on Elena (she compelled him to tell her nothing but the truth. What? It was justified).
He wants to stay with her forever.
"Forever's a long time.", she says, fear slowly making its way to her voice, "Are you sure?"
He sees the fear in her eyes (she makes a mental note to make stronger barriers next time) and gives her a kiss on the lips.
"I've never been more sure about something in my life."
"They'll stake me."
"Then I'll kill them."
She gives him a doubtful look.
"I will kill anyone that hurts you."
He loves her and, sometimes, she hates him for that.
"I love you."
She closes her eyes at his words and bites her wrist, letting her blood out. He takes her hand and drinks.
"I love you, too."
And with a kiss, she snaps his neck.