Elena gives as good as she gets with Klaus. With him, she is not ashamed or afraid to act the vampire and she takes full advantage of her abilities. It's not about love with them because neither is sure if they even know what love is at this point. It's about power and control and, in the end, it's about letting go.
It never lasts long with them, though. She always leaves. (She always comes back, but she always leaves, too.)
She's bad when she's with him and he encourages it with dark smiles and bloody kisses. She loves the way it makes her feel and she feels like if she's with him, she could be like this—like him—all the time and she would gladly sell her soul to him for the rush he gives her.
She does not know what triggers it, and she's sure he's not aware of it, but some nights when they fuck, he is gentle and slow and when she looks into his eyes they are so far away that she cries.
She leaves him then. (But she knows she'll be back.)