There were many different reasons for the cold war, I'll only include a few.
Firstly the breakdown the the wartime alliance; Russia(soviet union) and America never got along but they both put aside their differences to fight against a common enemy, Hitler. Their hatred for Hitler kept the alliance going through the war however when Germany was defeated, they had no reason to stay friends.
They also have different ideological beliefs. America believed in capitalism, where there was a free economy with privately owned businesses and people making profit. Whereas the Ussr believed in communism, where the economy is controlled by the state, businesses are controlled by the state and a doctor would have the same wages as a cleaner- They wanted it to be fair for people.
Stalin was angry that Britain and America kept delaying D-Day, believing it was a plot to allow Germany to weaken the Soviet Union therefore there was tension.
Germany; Stalin wanted huge reparations from Germany, and a ‘buffer’ of friendly states to protect the USSR from being invaded again. Britain and the USA wanted to protect democracy, and help Germany to recover. They were worried that large areas of eastern Europe were falling under Soviet control. It was difficult to come to an agreement.
There was resentment; Britain and the USA could not forget that Stalin had signed the Nazi-Soviet Pact with Germany in 1939. The Soviet Union could not forget that in 1918 Britain and the USA had tried to destroy the Russian Revolution
There are a few more I've missed out, but here's a good starting point.
