She was forbidden fruit because she's his teacher. This taboo made him want her even more.
Origin of forbidden-fruit
- From the story of the fruit of the tree of knowledge of good and evil, forbidden to Adam and Eve in Genesis 2:16–3:19
From American Heritage Dictionary of the English Language, 5th Edition
- With reference to Genesis 2:17: ‘But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.’ (KJV)