That's a question like "what is the meaning of life." Almost impossible to answer.
Resources, land, minerals: Some historic reasons, left over from when we were hunter-gatherers, is resources. A band of 20 people that only hunt and scrounge need a large territory, and intruders would not be welcome. This has evolved into nation-states warring over natural resources and fertile land.
Power: Some people crave power like a drug, and one way to have it is to be in charge of a large army, and use it to maintain (and gain more) power.
Ethnicity: "Those people don't look like us. They have weird hair and skin color. They hate us in return. Let's fight."
Religious belief: Substitute "look like" with "worship like" in the above.
There are more, I'm sure, but I think these are the big ones.