I did do the subject I wanted but the teacher was like "why do you want to do maths?" ahem because everything. Seriously though, I hate how my school becomes to involved in everyone's subject selection. ![Angry >:(](https://www.atarnotes.com/forum/Smileys/default/angry.gif)
I'm guessing you might go to a "good" school? In some cases, there's a conflict of interest created when a student wants to take a subject they're not good at. The school wants to maintain their history of high study scores and solid ATAR results, because these affect the school's ranking. While a student might be happy with accepting a 25 or whatever for a subject they're not good at but are interested in, the school and teacher involved might not be. I'm not saying you're only gonna get 25, but I'm saying you might mess up their precious stats.
It's possible that the above isn't true in your case, and your teacher is just genuinely concerned for you and wants to help you make good choices. But it's worth keeping in mind that sometimes your teacher will have interests that conflict with yours.