Fire and water is the most intense and delicate combination: one's passion with the other's emotional depth. Handled with care, there's steam — serious chemistry. Otherwise one feels extinguished and the other boiled. It all comes down to handling strong emotions.
One holds steady (fixed), the other flows (mutable): the fixed one gives the mutable roots, and the mutable teaches the fixed that changing isn't losing. Complementary — as long as neither caricatures the other as 'stubborn' or 'flaky'.