When you're selling a good or a service, you're exchanging one type for another, usually hand-in-hand. You buy food, furniture, etc. in exchange for money. There's value in the thing you're selling in exchange for a certain amount of money (which also has value).
When you sell money for money, this equation breaks. You're exchanging $1 for a different amount in the future, taking advantage of the fact that the borrower cannot pay you in the present. This is what defines interest and usury.
It's easy to get bogged down in the details to legislate or rationalize a certain action. Take a step back and see the destruction that interest based lending has caused throughout history, and in the present.
