he's going through the "no one understands me or my pain except you (the witch in this case)" phase/concept and she called him out on it and he got embarrassed because she figured it out
chuunibyou or eighth grade syndrome
The concept is most (if not all) teenagers going through at their age. Usually “no one understand me”, “I long for freedom”, “I want to free from adult’s restraints”, or “I’m not a child anymore so don’t tell me what I should do” phase. In other words, usual teenage rebellion towards adults.
After some time passed (usually college time, some high schoolers still has this phase), they matured and realized that they were being uhh... immature. They went to college, found a job, basically being an adult. Then they reminisced that period of time and called it as THE DARK PAST.
Wtf Can someone explain me about the Concept thing, I don't understand anything at all??! I'm so confused why is he blushing or what made him say it's embarrassing?