Guys, Why are older brothers so protective?

So my best guy friend and I were walking around school (it was break) and I see my brother and his friends walking by and he gave a death glare to my friend! He always does this when he sees my friend!! My friend tells me whenever he does it, he tells me it scares him abit cause he wouldn't do anything to hurt me, so he doesn't get why my older brother does that!
Guys, Why are older brothers so protective?
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