The key factors: / Break-down of the political system in East-Germany, caused by the decline of the USSR and emigration of East-Germans to West-Germany over other opening east-block countries. / Strong commitment to reunification of West-German politicians, especially Helmut Kohl.

The absolute key factor, the sine qua non was Gorbatchev's Perestroika. This kicked the crutches out under almost any Warsaw Pact satellite regime. It was absolutely clear that Gorbatchev would not intervene (like Russia did in Hungary 1956 and Czechoslovakia 1968) due to political change towards more democracy. So Poland, Hungary, the Czech went democratic. Second factor was the political pressure onto the GDR regime due to mass defection of their citizens. Since Czechoslovakia and Hungary were pretty much the last foreign vacation destinations for the people of the GDR, they used to massively exit through these countries: famous the storming of the West German Embassy in Praque, and the absolute stampede over the Hungarian-Austrian border. So the pressure on the GDR to permit German-German travel grew enormously, and as soon as the inner-German borders were open, the walls came tumbling down, literally! People crawled onto the Berlin wall and chipped away with chisel and hammer! The regime collapsed within days. Finally, the WWII allies, Great Britain, France, USA, and Russia/USSR agreed to permitted the two German states to reunify (France didn't look all too happy). Here the good personal relationship between Gorbatchev and Helmut Kohl may have played a key role.

karaoke

Ronald Reagan's peace through strengh policies put so much pressure on the Soviet Union it collapsed and could not longer support East Germany. It was just a matter of time but Reagan expidited it greatly.