From my understanding, and without going and looking it all up again, it's due to the nature of dogs as a whole.
It comes down to neoteny, the retention of immature characteristics. The floppy ears come from this, as do a lot of other behaviours. It's probably not something that was bred for originally, but while they were breeding dogs that would remain faithful, they were looking for immature behaviours, due to the nature of genetics, this also fixed other traits.