Why are men so protective over women?

Men seem naturally protective of women and I'm curious whether you think this is a biological instinct thing or something that society encourages and teaches them to be. Biological, i cant say but it just feels like the right thing for guys, but why?

Also this makes we question how much we treasure our women, wife or GF, I mean your friend steals your money and you scream and tell people he is a bad friend, but you hear that your friend is sleeping with your wife and you are looking for your gun or cutlass!!!! No Mercy 

Why are men like this?