Of all the civilisations of antiquity, the Egyptian seems to me to have been the most pleasant. The excellent protections which desert and sea provide for the Nile valley prevented the excessive development of the spirit of heroism which must often have made life in Greece hell on earth.

Otto Neugebauer – The Exact Sciences in Antiquity 2nd Edition, page 71.

In ancient Greece to be a citizen was to be a soldier. Marching alongside your fellow citizens was a privilege as was as a duty. But marching to where? The thing that soldiers exist to do is fight, so ideally to be the equal of the men who have gone before you you’re going to want to have fought in one battle. The money gained from booty is another welcome bonus. There’s also the matter of skills. If you’ve been practising something it’s natural to want to try it out for real. So what ancient Greece had was a lot of men who probably wanted a minor fight.

Making things worse, unlike Egypt, Greece never really unified from within. This meant that finding an enemy to fight was a lot less effort for the Greeks than the Egyptians. There were periods of division in Egypt, but ultimately the Nile valley mean that warfare would have to be pretty much one-dimensional. The Greeks on the other hand could sail away in a trireme and fight over all sorts of different coasts or islands.

So one of the reasons war was common in ancient Greece was that it was politically and economically rewarding as well as being easy to indulge in if you were a major power. It was built into the social system that to be militarily successful was the mark of a man. Thankfully we live in more enlightened times where a politician would never be enriched by a cynical war.