Lucius had to remain composed and cold for both their sakes, Buffy would only endanger herself and interfere with his plans. It was for the best that he push her away, she could not learn of the truth. It was strange really, how Daniel had deplored lying to her but had kept his species a secret from her. Lucius had far more to conceal from her and yet lying to her was no easier.
"I can and I will. Daniel died the day he was betrayed by his home and cast out, although that was not their intention. They intended to kill us both"