Fire
Xander hates Spike. He hates the smirk on his face, the mocking knowledge in his eyes, the way he crawls under Xander's skin and sinks useless fangs into his oldest wounds. He hates the twisted humanity of him, and the doubts it stirs up. But most of all, he hates his unnatural warmth. Vampires should be cold and dead. Their skin shouldn't burn to Xander's touch, sending strange shocks of heat through his body. He knows that it has to be wrong, somehow.
Spike hates Xander. He hates his clumsy humor, his eye-searing shirts, his stupidity in the face of danger. He hates the power the boy holds over him- a symbol of his debasement, this overgrown child keeping him tethered like a pet. But what he hates the most is the terrible heat of him, the warmth of the sun radiating from under his skin. He feels colder now than he's ever been before, and the boy's fire torments him with its nearness. He wants, but he can't have. He hates the craving within himself that makes him touch Xander more often than is necessary.
But for two men so starved for warmth, the infamous line is fine indeed.