Question

Why does the world fight over things constantly?

Answer 1

Everyone have their own selfish ends. Some countries wants oil, some want treasure, some are struggling to make themselves developed. Fights occur because of the difference of thinking between two countries, or even people, or because they want the same thing, and they fight over that thing, and the one who wins, take that particular thing as a 'reward' of winning. If everyone remembers this saying, then there will be no fight - 

"The world has everything for everyone's need, but not for everyone's greed"
    
                                                                                                    - Mahatma Gandhi

Answer 2

We as a world want power. Power over money, sports, land, religion, governamt. We do fight a lot but some countries want more than what they already have. Take Russia and Crimea for example. Russia is the biggest country in the world. And they want more land. Those are just some reasons why people would fight.