Why do wars start?

As long as there are human beings, there would be wars existed, I suppose.

Just think about family fight, if there are only one child in the family, there won't be fight since he's alone in the family. But 2 or 3 children, fights would be commonly-seen.

Human beings of diffrent nations, countries, may just like children of the earth, so although we are families, there would always be fights, wars, etc.
Maybe most of the times, just in another way.
Just personal idea,