If you want to see Mr. Colville debate the point in his own words, you can do so here:
To sum it up he brings up the fact that if you look in history (before guns are widespread at least) that there was barely a year when countries weren't at war, and that there is even an argument that we're taught about war wrong and that war is some strange catastrophe meaning someone messed up, as opposed to the default state that must be preserved. Basically, conflict is inevitable, peace has to be maintained.
Not As Devastating As We Think
There is a lot of poetry and art about the horrors of war. There is a lot out there on the devastation that war can wrought. Keep in mind where we are in history though. We have weapons of unimaginable destruction. Killing another person has never been easier. This doesn't disprove all of the rhetoric about war, but basically I'm arguing that mankind is more enlightened now - if only be necessity - than we were say pre-World War 2.
And the World Wars in a sense are the problem. We have this idea in mind of giant, massive wars where everyone in the world is involved. For our fantasy games this is reinforced with giant epics like Lord of the Rings, Star Wars, and even more modern works like Mass Effect. Game of Thrones could also be on the list, but when you think about it that's not actually 1 massive war. It's more like 3-5 smaller wars, and the reasons for those wars are wonderful because they're instigated over very small things in the grand scale of things.
Why Go To War?
For a war to happen there needs to be a reason, right? And that reason should be significant? Well, yes and no. I'm only going to cover a few reasons people in your world may go to war, but they should cover some ground. Also remember, these are instigating reasons, not necessarily the whole idea. So what do we have?
War For Honor
War for Honor can happen in a lot of ways. A country could go to war as a matter of honor because the other country is doing something egregiously wrong. But it doesn't have to be that big. Consider the War of Troy. What was the instigating factor there? Right, Paris stole Helen from Agammemnon, and so Agammemnon gathered an army and set sail to get his wife back. Maybe he had other reasons too, but there's a reason we have "Helen of Troy, the face that launched a 1000 ships."
Honor could also be avenging a slight. If a ruler or important figure has been insulted or otherwise dishonored, he/she could go to war to avenge it. Remember, the King is the country, so an insult to the King is an insult to the country. Or so the romantic rhetoric goes to get the soldiers to march. Perhaps a more fun way to say it would be "Don't tell a King 'yeah, you and what army?' because they can just point out the window".
War For Ambition
This is the more common reason for villains, but basically the instigator wants what the other has and is going to take it. However, this can be in a lot of ways. Consider that Nobles and Land Owners got that way somehow. Their worth is tied to their wealth. Their wealth is tied to how much land they own. And what's better than an ally with 50 shields that you can borrow? Just owning that land and the 50 Shields it can spare.
These wars don't have to be big. They can be as small as a few skirmishes and some light action, or as big as taking land and cities. The point is, something happens that makes an ambitious person see an opportunity to better their lot in life, and so they go for it.
War From Escalation
This cause is less about actions from on high so much as a series of escalation. You know the idea. A hits B with a fist, so B comes back with a Knife. This prompts A to come back with a bat. So B comes back with a gun. So A lights B's house on fire. So B salts and burns A's farmland. And so on and so on.
Basically, something bad - but potentially small - happens. Perhaps a cattle raid against a neighbor. But during the raid something goes wrong. Someone is identified. The wrong person is killed. Something. Now there must be retalliation. Because of the retalliation, the instigating side now must also get back at who they now see as the aggressors. Next thing you know, you're at war.
In L5R this can happen when two samurai have a fight. One lives, the other dies. They're from separate clans. Now the clan that lost a person has to avenge their clanmate. So they attack. To the first clan they've just been attacked, so now they need to counter attack.. Now, you have a war. Sometimes they start over a cup of spilled tea. Somethings the tea is spilled deliberately.