Answer:
Reason 1: The US and the USSR had different mindsets (communism vs. capitalism) when occupying the two sides of the Berlin Wall / Iron Curtain, which caused a lot of disagreement between the two super powers.
Reason 2: The US and USSR had nuclear weaponry and, due to the disagreements described in Reason 1, threatened multiple time to use them against each other to prove who's ideas were more powerful.
Hope this helps!