It’s her insecurities surfacing most likely. He told her he will make her a great mage and she didn’t believe him but still did her best to keep up but now she thinks that he’s a mage who could’ve took her as as his disciple to teach her more closely and yet he chose another arch mage. Is it because her talent wasnt as good as he wanted for his disciple? Something like that. PBut

I don't understand why that girl bother that her teacher became a disciple under woo heejae?