Yemeni unification didn't make much sense. Yemen wasn't like Korea or Germany. It had never been a unified political entity. North Yemen was its only little hick emirate and South Yemen was a British colony. Reasons for the merger were basically 1) South Yemen didn't have any money after the USSR collapsed 2) North Yemen was vaguely committed to pan-Arabism (they were part of the United Arab States with Egypt and Syria) 3) it was the momentum of world politics at the end of the Cold War 4) They both had Yemen in their name so why not?
Yeah, some people assume that the West and the USSR somehow "divided" Yemen into their respective spheres of influence as with Germany and Korea.
North Yemen (Yemen Arab Republic) was the successor of the Kingdom of Yemen, which emerged after the collapse of the Ottoman Empire when the various Peninsular Arab tribes were trying to consolidate power. The Kingdom of Yemen was basically just a part of the peninsula that Ibn Saud wasn't able to get a hold of.
South Yemen (People's Democratic Republic of Yemen) had previously been a British protectorate called South Arabia (they needed to control the port of Aden since it was a stop-over on the way to India and was located at the outlet to the Red Sea).
North Yemen was at least a semi-cohesive/coherent nation-state. South Yemen was basically just a bunch of tribes that had nothing to do with each other apart from not being loyal to the House of Saud or to the North Yemeni imam. Pretending to be Marxist-Leninist for a few decades was proof of how manufactured their national identity was, more than anything else.
