When those outside factors come into play (financial strain, child rearing stresses, family dynamics, illness, etc…), some people will get through it by doubling down on their marriage whereas others will turn away from their spouse and start to look for sources of happiness outside of their marriage. If both of you are in the first category, you find a way to get through those times and your marriage gets even stronger. If you are married to someone who falls into the second category, you may find yourself on a board like this one feeling the way you are feeling. It is what it is. Bottom line is that you can’t control what other people do and it is futile to try.
This is 100 percent accurate accept I would like to add that some don't cheat/leave or double down on their marriage and live a miserable existence together. Everyone knows at least one of these couples.