In one word, ignorance.
Many people of this world are ignorant. They are completely devoid of knowledge about other places, cultures, and ideas in this world, and then it branches off into discrimination, fighting, and hate. People cannot admit their own imperfections, and humans by design are not perfect. We project our problems elsewhere, and try to justify the things we do. This just fuels wars and hatred, making man turn on man. People end up not thinking for themselves, and just follow the leader blindly to make themselves feel more secure.
All of this, in my opinion, stems from ignorance. Our inability to sympathize with others, and our lack of knowledge about the other side of a conflict.