I'm actually somewhat the reverse of this situation.
I picked a subject because my mates were in it. In year 10, i had some retarded that idea since 4 subjects were the compulsory amount (i know how much of a bad idea that is now) that I'd pick only 4/5 and be just fine.
The previous day a teacher looked over our subject selections and told us we need 6..so i did the organised thing and picked a subject in the car...on the way there to hand in the form... I remembered a mate suggested i take up bio or geo with him.
I picked bio by a pure accident and turns out i actually like it (a lot)...it was a happy accident (lucky i didnt pick geo... the geo students got screwed over bad...there wasnt enough so they had to combine with yr10s doing it from other schools...on a wednesday afternoon...when everyone else gets to leave.. haha)