Hey Rutherford, my Dad says exactly the same all the time - I'm not doing a proper job, just dragging out being a student even more etc. I've found that it's easier just to say 'yes, you're right, when will I ever learn' etc then just get on with your own thing, rather than getting stressed out defending yourself all the time.
He has always wanted me to be an accountant, and it took me a while to convince him that I couldn't count very well.