When you borrow money from a bank, they borrow the money to lend to you. But they are able to borrow at a better rate than you can, perhaps 2% instead of 5%. So for every year that passes, they make (say) 5% - 2% = 3% of the remaining balance in interest. If you pay off the loan, they make no further money; if you pay down the loan early, they make less money in direct proportion.