Why did I get rejected for Care Credit dental loan?

I have around 500 dollars worth of dental work to do and applied to Care Credit but was rejected. I dont know why. Is it because I have limited credit? (I am 23). Or because I have outstanding student loans (I just graduated and got an extension on the re-payment). My credit score is 617. They suggested I can get a co-signer to increase my chances of being approved. If I use my dad, for example, do you think I will get approved? He has several good credit cards as well as a mortgage so he has a decent credit history. Thank u.
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Basically, a dental loan is a personal loan, and as such can be either secured (by pledging collateral) or unsecured (without the requirement of collateral). It is granted based upon your credit ranking and your ability to repay the lender. Dental loans are written by banks, credit unions, finance companies, and other lending companies. Many dentists often have a particular financing plan that they obtain for their patients through a specific lender. No matter where you obtain the funding from, being approved is dependent upon whether you are creditworthy and the amount you can be approved for is dependent upon your ability to repay the lender based on your income and the other financial obligations that you currently have.
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