Why do I get jealous when she works with this one guy?

JBird1000
I work with this very beautiful girl and she and I have been getting close to each other and talking to each other a lot. However, there is another guy who works with us and I heard she likes him. The thing though is that this guy has a girlfriend of 5 years and still going strong.

So my question is, why do I get extremely jealous when I see them working together? I know the guy, who is my friend, won't do anything against his girlfriend and this girl isn't the type to steal a man away from someone. It drives me nuts though, It happens every single time I see them working together and even when I hear about him texting her. Do I have a problem?
Why do I get jealous when she works with this one guy?
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