War is the continuation of politics by other means - Carl von Clausewitz.
Practically all state actors in international politics are rational. Certainly there are exceptions, e.g. Nazi Germany after Barbarossa, Fascist Japan after Pearl Harbor, etc. But even North Korea has some rationale for their actions.
Therefore, countries don't fight each other because they want to, but because they see it as a way to "get what they want", i.e. to achieve their political agenda. This is why most wars in human history have been rather limited; total war break out only when both sides know that absolutely no deal can be made.
Now, does this apply to Russia and Ukraine? Certainly not. Russia needs to make a deal some time as the war would make them look bad and likely result in sanctions, plus they just can't (and aren't interested) in occupying, not to mention annexing, Ukraine. Russia's priorities are keeping Ukraine away from the West (or better, pro-Russia), and keeping Crimea, as it is strategically important for Russia. Ukraine's priorities are maintaining its own territorial integrity to the greatest extent, and they'd be more willing than anyone to make a deal (they just don't stand a chance if war escalates).
Since both sides are "gentlemen" and are looking for a deal, why escalate things to the point of official state of war? It's just they couldn't yet reach a deal, but they'd have to arrive at one some time later. War is a very costly way to do politics, and they can't do it for too long, especially if both sides don't have too much money to spare.