Why did Ayato join Aogiri?

mamasaiko:

Just like Kaneki explained, Ayato’s actions are motivated by his desire to protect his sister

Not just due to the promise he did to his father when he was still a child

But because he is afraid to lose her too. 

That’s why Ayato’s violent tendencies increase as he grows up, because he identifies the death of their parents with the fact that both were weak, expressing that the only way to avoid being taken away your loved ones is to become stronger.

However, it is not until Touka decides to enter in the human world through school and Yoriko, that Ayato decides to leave his home and distance himself from his sister, traveling to other districts and getting in more fights.

The reason for this is partly because Ayato is aware that both worlds are incompatible, and begins to fear that just as it happened to their father, humans will betray his sister, and ghoul investigators will end up killing her.

This is the main reason for why Ayato joins Aogiri later, because in the eyes of ghouls, Aogiri’s goal is to destroy the CCG and eliminate the ghoul investigators, in order to create a world in which they can live without fear of being persecuted

In Ayato’s head if Aogiri wins and the investigators disapear, he would just need to protect his sister from ghouls, meaning that she could have a peaceful life, withouth him being afraid of her being stolen from his side.

However, although it is true that Ayato joins Aogiri to become stronger and destroy the CCG to protect his sister, he does it subconsciously, since he isn’t aware of the reason of his actions. This can be appreciate when we see that Ayato doesn’t seem to understand the explanations of Kaneki and seems to deny them

 until in his head appears a flashback of his sister after his father died

interrupting his line of thoughts and allowing Kaneki to hit him, for later submit him to his torture as a punishment for what he did to Touka.

Thus, Ayato joined Aogiri to protect his older sister, even though the motive of his actions were totally unknown even for himself.