Lelouch hasn't been throwing himself at Kallen, but he has been responding to her more and more.
Second episode of R2 he got worried when she was going to sacrifice herself against Rolo to let him escape. He reacted to her when she found him after he was all emo by telling her to fuck him. Then when she slapped sense into him, he was about to chase after her until Rolo showed up. A few episodes later he asked her if she would come with him after the war was over, and finally a few episodes after that he displayed clear worry over her when she was captured. He made a promise he couldn't necessarily keep, and did so on the spur of the moment without thinking about how it would sound to the other Black Knights present. A serious breach of his normal character.
He is after her in a fashion, its possible he himself doesn't know how much he cares for her yet, since love is a foreign concept to Lelouch. Evidenced by the fact he had to call Shirley up and ask about it. But he isn't throwing himself at her.