After WWII, why did Japan choose to become a less aggressive country?

Like Germany and Italy during WWII, Japan was led by an oppressionist leader, thinking of the country before the people. Therefore, you must consider that most of them were doing it with little choice, because they had to. They were also convinced it was the "right thing", at the time, but I suppose they also knew that they would suffer the same fate (nuclear bombing) again if they were to rule under such a government once more.