Why does he feel the need to be protective over me?

So I have this guy friend who will back me up during any fight I have with anyone like a attack dog. When he feels like I’m being attacked he jumps in and defends me. He gets pretty pissed off when I tell him I don’t need help too. Sometimes when we are hanging out together and someone calls my name, he will answer it like it’s his name and ask what do they want.
Why does he feel the need to be protective over me?
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