One habit in particular is guaranteed to strain a marriage, according to Kevin O'Leary, an investor on ABC's "Shark Tank" and personal finance author.
"The biggest problem in marriage ... is if one partner is willing to spend the other's money and has no regard for it," O'Leary tells CNBC Make It.
"You find marriages that start off on really good footing, but then you find out later your husband or your wife is a spend-a-holic, and they're spending way beyond your combined means," O'Leary explains.
"This is a problem you've got to nip in the bud, because it will lead to divorce with certainty, and often divorce with huge debts associated."
If your spouse or significant other is spending without your knowledge, they're committing "financial infidelity," a term used in a 2018 survey by CreditCards.com. According to the survey, 31 percent of respondents said hiding financial information from a spouse or significant other is worse than being physically unfaithful.
Before you make a financial and legal commitment to another person, O'Leary has some simple advice: Be sure to ask questions.
"Go out on a date and talk about my favorite topic, money," he says. "Ask, 'Do you have any debt? Do you owe anything to anybody? How much money do you want to spend? In the next few years, are there are certain things you've got to have, even though we collectively can't afford them?'"
Be upfront with what you want, and be open to listening to what they want.
"Start to understand what's behind the thinking of your partner," O'Leary continues. "If you're both in sync on this you're going to stay in love a lot longer and probably be more financially successful."
In his own marriage, O'Leary says he's on the same page as his wife, Linda. When they were starting out, they even had pizza and beer at their wedding to save money.
"You've got to talk about money," O'Leary says. "You've got to understand the other person's thoughts, because they may end up being a nightmare for you."
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Disclaimer: CNBC owns the exclusive off-network cable rights to "Shark Tank."