A really great question that should be asked more often. First and foremost, our patriarical human race is geared towards dominance and aggression. We are all in competition with each other. Next, you have governments or groups that have strong agenda's for dominance and aggression. Finally, but probably not the last, is that individuals and groups in power will always have a large pool of economically disadvantaged indivudals that are "willing" to become obedient to fanatical soldiers.