Answer:
They were both spying on each other and they didn't like each others way of running the country. what made things worse was how they both were occupying Berlin. Russia was trying to use Germany to show how 'great' communism is but they were failing. because of the US being in west Berlin citizens could see how bad it was and this caused Russia to put up a wall. President Kennedy had used this to is advantage and said they may have problems but they have never had to force their citizens in. Even during world war 2 they were becoming.. not so friendly with each other. the only reason they tolerated each other by the end was to stop the larger threat that was Hitlers Nazi Germany.
I hope this helped!
<3