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 initiates (cardinal), the other sustains (fixed): done well, it's the formula for projects that start AND finish. Done badly, one pushes change and the other digs in. Negotiation is this bond's muscle.