Why does the world fight over things constantly?

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