Come on man, ease up on me. I just admitted that I couldn't remember why it was time to unify Syria-Iraq again, and now you want me to explain why Czecho-slovakia was unified?
I'm an ooold FReeper, but, okay, okay, I'll try .... they were unified because .... because .... well, because the two countries were .... well, they were .... hmmmmmm, exactly why were they unified again .... I seem to have forgotten .... only for the moment of course ... so could you tell everyone why?
Very simple really. During the Balance of Power period of European history, overly small countries generally got gobbled up by larger neighbors.
In the settlement of the Napoleonic Wars what is now Belgium was combined with Netherlands and Norway with Sweden to make more powerful nations that might resist aggression by the French or Russians.
After WWI the same was done when they broke up the Austro-Hunagrian Empire. The various "south Slav" ethnies were combined to form Yugoslavia, and the "north Slav" equivalent was formed by the Czechs and Slovaks.
None of these shotgun weddings worked out all that well.