You've gotten this far in math, so you're probably at least somewhat logical. Maybe you play chess or dabble in philosophy or know that riding your stationary bike to school isn't really an option. You make some sense (although your parents might beg to differ).
That's a wonderful starting place, but geometry kicks it up a notch. It asks you to appreciate the intricacy and artfulness of a well-reasoned argument—and even come up with your own.
And no, trying to convince your parents that tomato sauce makes pizza a vegetable doesn't count as a well-reasoned argument. Just because Congress agreed to it doesn't mean your folks will.