"Elena isn't Katherine," Damon pointed out. He didn't turn to face his brother, afraid that perhaps something showed in his demeanor, something he didn't want Stefan to see. "She's nothing like her, except superficially."
Was that the fascination, perhaps? Elena had Katherine's beauty but none of her faults? Oh, Damon had no illusions about Katherine Pierce. Willful she was, selfish and self-centered and unscrupulous as the day was long. And exciting and quixotic and energetic as well, if he were to be honest. She had a pull on him which lasted even to this day, long after she had ceased to exist.
Maybe even further honesty would dig deeper into the roots of his fascination. Maybe there was a touch of turnabout is fair play. Stefan had interfered in what he shouldn't have - namely Damon's relationship with Katherine. Was this a little payback for that? Even Damon wasn't quite sure of his motives.