There are at least three main reasons for this change.
First, there was the fact that Nikita Khrushchev had, by 1955, had some time to consolidate his power in the Soviet Union. There had been a relative thaw after Stalin’s death as Khrushchev was new in power. By 1955, he was more solidly in place and was willing to be more aggressive towards the US.
Second, there was the fact that both sides were starting to worry about the other’s actions in more parts of the world. This was truly becoming a worldwide Cold War. There were crises in the Cold War in such places as Iran, Guatemala, the Suez, and Cuba. As these things happened, each side was more and more worried that the other was trying to expand more and to achieve world domination. This led to worsened relations between the two superpowers.
Finally, there were technological advances that increased tensions as well. The development of missiles that could be used to carry atomic warheads caused great anxiety. This led to the biggest crisis of these years, the Cuban Missile Crisis. The changes in technology changed the strategic logic of the Cold War, also changing the relationship between the superpowers.