It is so called 'terrible two'

This is part of the phycological/neural development. That's the time the little person define what I is. It will pass.

They will have it again at the age of 8 to 10, then the pubic period 13-16. That's part of the brain development, the chemical and homonal changes happening inside them.

Basically the little people 'learning the rope' in this world, and positioning themselves.