In it's simplest form, to make a loan, you have to have the resources to loan out to begin with. Furthermore, dereliction of debt is expected. There is no guarantee that people will repay in a timely manner, and missed payments/discharged debts are not uncommon. As such, interest rates can serve to create a sustainable system by helping to cover these debts.
Discount any consideration of their ability to repay, and what you give is no longer a loan, but a gift. A gift at the cost of others that you loan to.