system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is dating a friend's ex-partner so looked down upon?

Is it a cultural thing or is it across the human spectrum? Is it a personal preference? I actually can't wrap my head around it and was hoping for a simplified explanation.
gpt
Suggests you were daydreaming about your mate's partner when you should have been supporting your mate - i.e. conflict of interest. Also, it sucks for your mate who has to think about his/her mate being with their ex-partner and also potentially having to interact with them whenever they hang out with you.