You are completely right, if we all lived without war we would all be completely fine. But that's not going to happen because we're human. We have too many faults for life to be stable. This doesn't mean mankind is going to get obliterated from the face of the earth by our mistakes, but it shows that we are forced into a perpetual state of conflict.
1) Paranoia. Attack your enemy before they attack you. Whip out the first punch and knock him out in one go. Quite a few countries in history attacked other nations because they FEARED getting attacked themselves. Therefore, why wait to get attacked when you can throw the first punch now? This was in the case of Rome. The Romans were scared of being attacked by foreign nations. In order to quell such fears, the empire chose to attack neighboring nations to take away these threats.
2) Greed. You have something I want. We negotiate but you want things in return. Well, I don't want to give you shit so I'm going to fight for it instead. Pretty stupid but this is a common reason why wars start. WW2, the Sino-Japanese Wars, Operation Desert Storm, these were wars for land, resources, and living space(no joke). People have negotiated before these wars(not specifically the ones listed above) but negotiations that fail will most likely result in war. If negotiations take to long, war. If they want something in return, war. War is a quick but very risky solution to gain some kind of luxury of necessity. Some times it is justified but most of the time it isn't.
3) Prejudice and Differences. I don't like the way you talk, walk, or live your life out. It pisses me off so I'm going to beat you up because of it. This sounds horrendously unreasonable and irrational, but it happens even now. History shows that war allows people to eradicate things they don't want or people they don't like. The Crusades, for example, was a massive jihad against Muslims and Jews. Many were killed, raped, and tortured by the attackers. Their excuse? They were not Christian. Same could be said about ISIS and other religious terrorist groups.