Why in the 1950s-1960s the relations between the USSR and the United States either deteriorated or improved

Because all the time there was an unspoken rivalry between these two countries. And even after Stalin’s death, when Nikita Sergeyevich Khrushchev came to power in our country, whose plans, first of all, was to improve relations between the USSR and other countries, he could not maintain equal relations with the United States, because;
1) After a visit to the United States in 1959, Khrushchev returned in awe of their success. in agriculture, after which he ordered to immediately sow our fields with corn.
2) In 1960, the Americans dared to send their U-2 reconnaissance aircraft over the territory of the USSR, ours shot it down with a missile. There was a terrible scandal, relations between the USSR and the USA deteriorated greatly.
3) While vacationing in the Crimea, Khrushchev drew attention to the fact that even the beach is within direct reach of American missiles located in Turkey, Nikita Sergeevich came, quite frankly, from this into a rage and decided to place his mid-tier nuclear missiles in Cuba thereby endangering any US city that could now be razed to the ground in seconds,
4) US President Kennedy, when he learned about the installation of missiles in Cuba from the data of aerial reconnaissance, also flew into a rage, he “tore and threw”, announced the establishment of a naval blockade of Cuba. Now no military supplies could enter the island. This was a clear violation of international law, but Kennedy was even ready to engage in open military confrontation.
5) Only after long behind-the-scenes consultations Khrushchev and Kennedy reconciled, managed to find a compromise, they say, the USSR is removing missiles from Cuba, and the United States is refusing military intervention in Cuba’s affairs and is withdrawing missiles from Turkey. It was in the light of all these events that relations between the USSR and the United States were unstable all the time, sometimes improving, sometimes worsening.