Why would guy coworker get jealous of a girl talking to another guy?

He shows signs of being attractive to a girl,, but he knows she likes him, yet he doesn’t do anything about it. It’s been going on several months.

However when she talks to another guy colleague (who she is friends with), he gets jealous and gives her the cold treatment.

Why is he even bothered? Why does he blow hot and cold?

Why would guy coworker get jealous of a girl talking to another guy?
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