Severus is fire. He is warm-hearted under the brutal façade that he wears like a cape. He is also passionate, but that is not something many people know about. His emotions burn brightly, sometimes too much so, and he hides this secret fire away from the world, in a corner of his being that only he knows how to access.
Lucius is Ice. His demeanor towards anything that dissatisfies – or appears to dissatisfy – his is chilling. He is perfect, as if one touch of fire will melt him. This is why Severus kept his feelings secret, fearing that even a little fire would melt the ice. But he forgets that water douses fire just as surely as fire melts ice.