I just want to give a name to this for whoever want to Google more: novelty seeking.
I think it's part of how we're wired. Some people more than others. To seek that dopamine hit again.
Yeah, the trick is to recognize you are a person that likes that and then try and have that risk/excitement sustained in your own relationship/s. A tip I've heard for those that need it: Always be dating your spouse. I'll leave it to the reader to google the ways to do that ;)
Some people value an exciting encounter over their current relationship, people can have an addiction to the rush or have felt numb for too long.
Over simplifying - there's nothing wrong with their current phone but some people will rush and buy the latest to get the buzz of a new gadget.
They weigh the chance of getting caught and damage it will cause as lower than the excitement and fulfilment of a new fling.
We've all been there, seeing someone attractive from the opposite sex and having thoughts of an encounter. Whether you chose to act on it or not depends on your integrity and empathy. Some people have more than others.