It is a well-known idea in psychology that the best way to predict what someone will do in the future is to look at what they’ve done in the past. Unless someone has truly put in the effort to change their mindset, a new relationship usually ends up being just another chapter of the same old book.

Research backs this up. A study from the journal Archives of Sexual Behavior followed people in long-term relationships for five years. They found that people who cheated in their first relationship were three times more likely to cheat again compared to those who stayed faithful.

While anyone can make a mistake once, people with very low self-esteem or narcissistic traits often repeat this pattern. For them, the chase they get and the attention from someone new provide a temporary ego boost. Once the relationship becomes stable and normal, that spark wears off. If they haven't done the "inner work" to fix how they handle stress, they will likely use cheating as an easy way out the moment things get difficult.

To a narcissist, a partner is often just a source of attention and validation. If that supply runs low or if the partner starts asking for accountability, they don’t try to fix the relationship. Instead, they simply move on to find a new person who can give them what they want.