Jealousy is one of the oddest emotions we experience as humans. It does not matter if there is a shred of evidence for the intensity of the feeling of jealousy. Our system does not differentiate between seeing your partner with another person and believing that your partner is with another person. This is similar to experiencing fear. Our nervous system responds the same way to the perception of fear and danger. Example: If we read a post on social media stating that an asteroid is about to hit Earth in our town, our nervous system will create hormones and thoughts to protect us. It turns out that someone posted this to annoy other people, and there was no threat. If an asteroid hit Earth in our town, the body and nervous system would still have responded the same way. Since jealousy is a form of fear, we have the same kind of experience. Example: If we feel threatened by our partner being contacted by their ex, we may feel a surge of jealousy, knowing they are connecting. Some people create all kinds of narratives inside their heads around why they are connecting and what it means to our relationship today. They may begin to distrust them and take steps to find evidence that there is a threat. And while doing so, we may view normal situations as evidence, confirmation bias. Now that we have "evidence" we may confront our partner and have trouble sleeping at night. And as the process escalates, our brain may create more "evidence" to confirm our fears. For some people, this can hijack everything else happening in their lives. Eventually, we find out that their ex was contacting them to let them know that a family member they were close to was killed in a car crash and the funeral service is Friday.