I've noticed these locks on bridges in many places in Europe and in the beginning they baffled me, I must admit. Apparently couples put them up and throw away the key to symbolize eternal love. Now the locks have come to Copenhagen, though not as mainstream as in i.e. Eastern Europe. These few locks have been placed on the bridge across Nyhavn, but who knows, it might catch on here in the next few years. We have enough bridges to last a few lifetimes.