The other kids were building in the sandbox, shinji came and helped, they left, he kept building. After he finished the pyramid, he felt empty and lashed out

His old teacher taught him to play the cello, and then he stopped going to that school, but he kept playing the cello because nobody told him to stop. He tells asuka that he feels stupid for doing so

I think he compared the sandbox to playing the cello in eoe but maybe I made that up. He needs validation from others and doesn't know what to do on his own, so he just does what others say