My dad's good friend is a marriage counsler. He has some wild stories, but he mentioned some of the differences from 30 years ago to today are communication, people will text rather than talk in person. Like they will air their grievances via text on the way to work rather than the night before or morning in person, then not discuss it at all later in person. Thats wild to me. Another was ease of divorce, everyone doing it and thinking they want that single and free lifestyle back. And then sports, families being strewn all over the state/country for kids sporting events and never having any time together. It leads to disconnect between couples and that obviously leads to infidelity.
Then there are the timeless reasons like infidelity, one or both giving up on appearances, falling out of love, money, career.