The simple answer: Because you believe every other option can yield greater losses...
So the thinking is; Lend it to France you'll get your money back. Put your money in a bank and there's a chance you won't get it back if the bank goes bust.
Investors also believe that rates will continue to go more negative, in which case they will actually MAKE money. It's a momentum play which will eventually reverse but since the Bond market is extremely liquid, it's a bet that can quickly be unwound.
Another theory floating around is that if the weaker countries of the euro leave (e.g. Greece, etc) and the core keep the euro, then the value of the euro will actually rise. So by putting your money in French or German bonds you would also be securing cash in French or German euro's. - Thinking the unthinkable on a euro break-up - Gavyn Davies