Marriage is an investment that pays dividends if you pay interest.

Monkhouse frames marriage in the language of finance: a long-term investment that can yield “dividends” (happiness, stability, companionship) only if one continually “pays interest,” i.e., keeps putting in effort, attention, and care. The joke hinges on the double meaning of “interest”: both the cost of borrowing money and the emotional engagement required to sustain a relationship. It implies that marital rewards are not automatic or guaranteed; they accrue through ongoing contributions—small, regular acts rather than one-time commitments. As with a financial investment, neglect leads to diminishing returns, while consistent maintenance compounds benefits over time.