Why are kids particularly fussy when they're 2yo?

This is the age... ...When they have learned "can and can't", but not "may and may not".
Emotional conflict. We run to infants when they cry & do anything to sooth them, which sets them up to treat us like servants.As the approach 2 they expect us to stay close by but don't want us to interfere with their play.We begin to leave them to their toys (to get stuff done) & they fuss when we don't stay.They fear separation but want control.Be patient and help them understand you are nearby & they can play alone.