It's human survival instinct to fear. It's like your body anticipates and perceives some sort of "war" is going to happen where your safety might be compromised. It teaches us to be humble and respect everything around us.
Fear is an excellent motivator, it increases your adrenaline level which improves your senses and reflexes, it allows your brain to focus on the situation at hand. The trick is learning how to control it, if you don't it can cause you to panic, when this happens you become a worthless sniffling meatbag and your chances of survival begin to drop rapidly. Enlist fear as an ally, don't allow it to become an enemy.