Mostly out of necessity.

France, Holland & Belgium were conquered quickly because they had to be. Had von Mansteins plan not been approved, it's possible the war would've gone much like WW1, with a series of fronts that would rarely change much, and anything lost quickly being retaken. And they wanted them under German rule because they wanted to knock them out of the war and force the British to agree to peace.

Norway was conquered because the Germans feared the British were going to use it as a base for Naval Operations and a potential invasion.

The invasion of Greece was for much the same reasons.

Poland was invaded because they refused to give up the tract of land that would connect Konigsberg and the surrounding area with the rest of Germany.

Anything east of Poland was down to Hitler's desire to conquer Russia and not at all necessary.

Source: B H Liddel Hart The German Generals Talk.

Have a look at Speer's Inside the Third Reich and the Third Reich Trilogy by Evans.

Long story short, much of this was driven by what the Japanese would call "victory disease", especially on Hitler's part. Remember, between 1933 and 1941, Hitler had brought the NSDAP to power, shredded the Peace of Versailles, annexed Austria, subjugated Czechoslovakia, dismembered Poland and then turned the much vaunted Maginot Line into Swiss cheese.

By New Year's 1941, Hitler had effectively become the master of Europe, with no one left to fight him except for a wounded Britain. (Thanks, Molotov-Ribbentrop pact!) The flip side, however, is that now there was no one inside the German government who could talk Hitler out of his crazy idea to invade Russia and defeat the Soviets in a quick war.

It was that hubris that led him to attack Russia, and ultimately into oblivion.

The immediate goals of the war were not a land-grab, but the neutralization of Germany's enemies. For the sake of reference, this is the map of Europe (from Wikipedia) showing territories annexed by Germany (Sudetenland, Austria), occupied by Germany (Northern France) allies and puppet states (Hungary, Croatia) and areas under enemy control.

The immediate goals were to neutralize enemy armies and establish puppet states or allied countries (which were very often the same thing).

Austria-Hungary, the Ottomans and Tsarist Russia fought different kinds of wars, one were mobility of armies and supplies were extremely limited. None of those empires ever faced a situation in which an enemy country could bomb their capital with bomber planes. Tanks and mobile divisions meant that huge swaths of land could be conquered within hours.

Germany's idea of war was to reach the capital, depose the government and install a military occupation government or puppet state. The entry of the United States into the conflict made this a very difficult task, however, Germany did abandon the plan (Operation Sea Lion) to invade Britain and Germany failed in the Battle of Moscow (among others, most notably Stalingrad).

Capturing enemy territory without defeating the enemy would just set Germany up for a counter-attack.

I oversimplify when I say "depose the government" - there are many other objectives, such as forcing a surrender, creating a local ally, cutting off supplies from the active enemy forces, disabling or seizing their production of arms, etc.

Germany's advantages of readiness, speed, the element of surprise and catching the enemy unmobilized or unprepared would be completely gone. Had Germany waited, its enemies would have mobilized and attacked and Germany would have to defend huge parts of land, instead of attack, which was their strength.

Even if Germany had a very strong defense, a long, protracted war of attrition being surrounded by enemies was expensive and unwinnable. The technology of the time meant that such a war was highly unlikely.

Furthermore, Germany's main allies - Italy and Japan, involved Germany in a wars that had nothing to do with their immediate or eventual objectives. Japan's war with the United States and Italy's invasion of Greece were not part of Germany's objectives. Germany did not try to conquer the US or Greece for territorial gains, for example.

But taking that out of the equation, the war started when Germany invaded Poland. Germany could not stop at that point and be satisfied with its gains because they were at war. They could not put the genie back into the bottle. War with Britain and France (among others) was inevitable and the only question was how much time they gave their enemies to prepare. Collective agreements and treaties ensured that one-war-at-a-time was not a possibility.

Short answer: the Concert of Europe. The 18th and 19th century witnessed every country desiring European hegemony facing a united coalition of it's rivals. Even if Germany had expanded at a slower rate, eventually they would still face a united coalition of rival European powers (and probably the United States as well). The German strategy at the outbreak of World War Two was actually sensible- a series of knock out blows to quickly eliminate their potential rivals. They tried this at the beginning of World War One, and it came very close to succeeding.

For more on the topic you might try "The Struggle for Mastery and Empire in Europe, 1848-1918" by Alan J.P Taylor.

It seems that the Nazis doomed themselves by seeking to conquer so much so fast. What was their motivation for this?

Hitler did not necessarily anticipate a 'world war' to erupt on his invasion of Poland - what I'm saying, is that events as happened did not always go to plan for the Nazis.

They actually did not seek to expand as quickly as they did - it's more appropriate to view the evolution of the war as a series of accidents and improvisations.