system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening within a childs development during the 'terrible twos' stage?

My nephew has recently turned 1 and is possibly one of the best behaved babies I have ever seen. He always seems to be smiling and laughing, hardly ever cries, and generally just complete chill 24/7. 

My mum made a comment the other day saying that he's probably going to be a nightmare during his '"terrible twos". It is often said that when a child turns the age of 2 they start 'acting up'; tantrums/crying, hitting etc.

What is actually going on in terms of child development that causes many young children to act this way?
gpt
The child's brain has now developed enough to attempt independent decisions and choices. However, to practice this, the child must disagree with adults (parents etc.) and assert some different opinion. Because the skill is very primitive and new, the child has very poor judgement about when to disagree and when to agree. Also, because the child's mental and physical skills are still developing, failure is extremely common, leading to frequent frustration and anger.