Why do people fight over stupid stuff?

I'm a 14 year old girl, and let me tell you, this pisses me off more than anything. Seriously, is it necessary to fight over something that won't matter five minutes from now? My sister just started screaming at me for leaving my water bottle on the floor. I picked it up, but are you kidding me? The way she screamed at me is the way you would scream at someone for punching you in the face or something. I've noticed this with other people too. It's not like anyones going to die because of one person forgetting something. Obviously if I left the oven on or something, I could see why someone would be mad, but if you're going to start a huge argument because of something as stupid as a water bottle, you seriously need help and have to go to anger management classes. Just calm down people.