How did the interest rates on my car loan go up?

Initially I was pre-approved at my bank with a fixed interest rate at 6.24%; for the loan amount of $14,000 just under me without a co-signer. It turn out it cost more for the car than I've expected. So the finance officer at the dealer said my bank won't give me more than what I've asked for. I thought that was weird. So I went with their bank with my dad as the co-sign. The dealer did his thing and came back with a interest rate at 10+%! Now just to let you know, my dad has good credit. My dad has never miss any mortgage payment or any of the two car payments he has. And I my self have a credit score of 663 which is good itself. I've have been making payments for my previous car for more than three years on time. What should I do?