I think infidelity, IF you define infidelity broadly, meaning not simply adultery but a general lack of commitment to one another. Lack of devotion, care, and consideration for your spouse leads to indifference, then apathy, and eventually, contempt.
Finances can and do put a strain on a marriage, whether that be too little money or too much money, but any couple can survive financial strains if they are committed to each other.
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