I hate to assume such a horrible thing, but, fun I believe is the main reason. The mentally disturbed person who begins the whole thing can't possibly have the best interests of people and the environment on their mind, but would rather consider it to be a bit of a thrill, a challenge. Feeble reasons to kill people can easily be thought up, and the prospect clearly cause little distress to the leaders' minds. Being recruited for war has been described as "exciting" and "patriotic" but, as your question implies, nothing, nothing that was ever gained from war could make up for what was lost. No reason, just male killer instinct.