Why do I get jealous over my female colleague although I have a girlfriend that I’m happy with and plan on marrying?

Anonymous
I love my girlfriend and I’m currently looking at engagement rings. She’s great and I’m happy with her.

I have a co-worker who is pretty cool and she seems to get a lot of attention from guys. Although, we’re never went on a date or had sex (obviously) I can’t help but feel jealous when guys express interest in her or interact with her. When she would go on dates, it made me upset and jealous. I recently found out that she has a boyfriend and now I’m very jealous. Last night, I spent over an hour trying to find information on the internet about this guy. She I kept questioned her about him and their relationship, she got defensive.

Why am I jealous?
Why do I get jealous over my female colleague although I have a girlfriend that I’m happy with and plan on marrying?
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