Why would I lend you money? If you just promise to pay me back then I’ll be missing out on money now only to end up in the situation I was already in before I gave it to you. (Actually worse if you take inflation into account). And actually you might not even pay me back at all, what if you die? What if you can’t pay it back?
Now if you said you’d pay me back + extra now I actually have an **interest** in giving it to you.
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