Why can guys be so protective and possessive of their girlfriends?

Don’t get me wrong I love it when a guy is protective of me but I’m situations where I’m simply talking to other guys and he gets possessive all of a sudden I wonder why? It’s just a friendly conversation it’s not like their was any flirtatious motives going on. Surely a guy can see there’s no cause for concern so why the sudden need to shield me away? Have a spike of jealousy? Are they not able to read the room well enough?
Why can guys be so protective and possessive of their girlfriends?
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