Wars dominate Western European history, but it's hard to imagine two Western European countries warring today. Why is that? Is it because our economies are more intertwined? Because mass-communication has made people from distant places more accessible, less scary, and more human? Or is it because history is taught in such a way to overly emphasize periods of war, giving us the illusion that war was a near-constant?
Hopefully someone can provide a more nuanced and sourced answer but to such a vague question:
Wars were and are common to every part of the world, not just westerns Europe. Everywhere where political borders exist wars are common through history.
War is not unthinkable now at all. The political and economic boundary has shifted from the west side of Germany to the right side with the recent Ukrainian crisis that should be all the evidence one needs. A few years before WW1 famous politicians and writers were speaking of the end of warfare and how new global trade made war simply economically unfeasible.
To really answer the second part of your question though it's that post-WW2 France, UK, Germany, and the Nordic countries have accepted common but not exact political and economic systems. There isn't really any incentive to fight, especially being allied with the USA. What you're asking is a huge and complex question but that's a very brief summary from my knowledge.
I come from a political science background with a focus on conflict studies, maybe I can shed some light on this. Especially on the post-WW2 situation.
There is a number of competing theories as to why great power war is declining and none of them can give us THE answer. This is specifically about wars between big powers, not small-ish border disputes or proxy wars.
The nuclear peace theory. First formulated in Kenneth Waltz' article “The Spread of Nuclear Weapons: More May Better”, it assumes that the sheer destructive potential of nuclear weapons is enough to deter those who have them from going to war against each other and their allies. This theory doesn't preclude all wars fought against or between nuclear powers, but those that threaten the very survival of the state. Pakistan and India have an almost constant border conflict in Kashmir, but neither of them would feel threatened enough by these actions to risk nuclear war. Some say that during the 1973 Yom Kippur War, Syrian tanks were held back from pushing further into Israeli territory by fears that in doing so they would trigger Israel's nuclear option. In essence, nuclear weapons remove the connection between victory and survival. With nuclear weapons, you can win the war and still not survive to reap the spoils. For this theory to work, either only one state can have nuclear weapons OR both states need true second strike capabilities.
The Democratic Peace theory. It claims that democracies don't wage war against each other. Mainly because it is harder for countries with similar values and political systems to see each other as hostiles, that political leaders who come from a democratic background are more inclined to engage in negotiations and resolve conflict through peaceful means, that the leaders are publicly accountable for the inevtiable losses of war and that democracies tend to be richer and have more to lose.
International institutions. NATO, UN, EU and others give states a myriad of options to resolve conflicts peacefully. They bind states together and further understanding. They have rules that prohibit armed conflict and promote peaceful resolutions.
Globalization. The world economy is so intertwined on a global and regional level, that from an economic standpoint, waging war against any of the 'big players' in the world economy or against your neighbours is going to cost more than you could possibly win.
Morality and international law. War used to be every state's right and a normal way to resolve conflicts. Morality and international law have now changed in a way that prohibits war for any reason but (collective) self-defense. How that change of heart came around again is debatable, but it is fact that it is reflected in international war and that now, as opposed to the interbellum period, the prohibition of war is taken seriously.
As I have said in the beginning, neither of these theories is likely to be the one that explains it all. They all have their merits and flaws and personally, I believe it is a mixture of all of the above with a heavy emphasis on nuclear and democratic peace.
Have there been any conflicts in post-ww2 western Europe that may have escalated into war if not for the factors previously discussed up thread?