Have you heard the story about the man who bought a U.S. Treasury Bill for $1000 knowing before hand that he'd only get back $995 at the Bill's maturity. "What kind of idiot would do something like that?", you ask. It's irrational, it doesn't make sense. Give someone $1000 knowing you're going to get back - not more money in the form of interest but less money. You're paying someone to take your money.