Where does the rationality of a 3 years old boy come from?

sooooooooo cute!
As an answer to the question stated in the title, I tend to say, it is the power of nature plus the up to date peaceful human condition.
But why does not everybody bahave in the same way? Generally speaking, it is the power of social influence, which is based on different individual psychological reactions toward certain circumstances.
Underneath all individual characters, inspired genes and existing living condition play important roles.