Why do single people get so jealous of married couples who are happily married?

Trigger warning but I don't care about what people say vs how they actually respond. You can tell single people get jealous cause of their facial expressions when they see two happy couples holding hands or anything more than that. I understand PDA is an ick for people but then why do they get repulsed by seeing them hold hands or sending each other love gestures like love cards, and heart symbols using planes, birds, etc.

Cause they want what most people don't have which is true love and that's rare
Some other reason
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Why do single people get so jealous of married couples who are happily married?
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