This applies more to men but also to women.
You are happily in a committed relationship except for one little/big thing. You don’t “rock” your partner, you don’t “light them up”. They love you, you are the one, your are a comfort to them, there is intimacy, children, solid finances, respect, but …
In the back of your mind you know or suspect you are not “wow factor” which could come along at any time from someone else. Then what? It might not be the end, it might not even be a classic emotional affair.
Someone else captures their attention, they “long” for someone else, they have another person “on their mind”. You might even sense it. They can’t always hide it.
Now, can you handle it? As always, be honest with yourself!
Should I stay, or should I go?
There are many examples of this dilemma presented in movies. For example, Owen Wilson in Meet the Parents and Netflix Sex/Life.