It's Valentine's Day, when even reasonable people wander around, engagement rings at the ready, blathering about how "love is the answer." Snap out of it! Love, in fact, may be the problem. Passion often blinds sweethearts to the fact that matrimony is, at bottom, a contract. Figuring out how that partnership can prosper is critical for a successful union. Yet financial differences rank among the greatest sources of marital misery, in part because talking about money before you tie the knot makes many couples uncomfortable.