Why people in relationships thinks the grass in greener until their partner leaves?

My friend was in a relationship but she’s been talking to another guy, she said she wants to be with him. Now that her boyfriend broke up with her she’s not talking so much about the guy she was crushing on, Is this a case of you don’t know what you had until it’s gone? Why is it easier for people to think the grass is greener on the other side while in a relationship but can see things clearer wants their partner leaves them?
Updates
6 mo
😂😂😂😂. I can’t help it she’s my friend and it ended already
Why people in relationships thinks the grass in greener until their partner leaves?
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