To what degree was the escalating U.S – Soviet tension in Europe the result of excessively defensive fears in the minds of Truman and Stalin?
I personally think that fears of the Soviet Union were entirely justified. After all, at the end of the Second World War, the Soviet Union was the largest and most powerful dictatorship on the planet. It had imperial ambitions and, most important, it had atomic ambitions. Stalin's treatment of his own people would have been cause enough for anyone in the U. S. to be justifiably concerned about the Soviet Union under Stalin's leadership.
You have to consider Stalin's fears because not only was he a dictator, but he ran the USSR as a cult of personality. Stalin's fears were Russia's fears. As for Truman we have to consider the balance between how much a president affects the feelings of the US with how much actual personal power he has.
At the beginning, however, Truman was oblivious to some of the politicking that had gone on at the Yalta Conference, and when he met Molotov, Stalin's underling, he took a very hard stand on Poland, one motivated by increasing fear and mistrust of the Soviet dictator as well as the knowledge that the US had just tested an atomic weapon successfully at Los Alamos. I would say it was in part fear and simply different perspectives and strategic goals that soured both parties on the Grand Alliance in World War II.
I don't think it's right to say the tensions were due to the leaders. That might be true in the case of the Soviet Union since Stalin was such an autocrat, but US politics drove Truman's fears just as much as Truman's fears drove American policy. There is no way, for example, that Truman could seek rapprochement with the USSR during the McCarthy Era.