In the old days, a lovelock was a lock of hair given to someone you love. Nowadays, though, it makes a lot of people think of the locks attached to bridges, originally in Italy or perhaps even Serbia, but now increasingly all over the world, to symbolize eternal love.
The Pont de l'Archevêché (Bridge of the Archbishopric-- the bridge is just to the east of the cathedral of Notre-Dame) is the favored spot for this touristy vandalism in Paris, along with the Pont des Arts. You can see from a distance that the bridge's railings are dark. Those are all locks.
Here's what they look like closer-up. They make the bridge considerably heavier.
For a long time after the fad started in 2008, the city of Paris removed the locks regularly. They are bad for the bridges and monuments they're attached to and pose a danger to people on the boats that pass underneath. But the locks have become a tourist attraction and impossible to stop, and the mayor does not want to forbid them. Vendors have sprung up selling locks specially made for the purpose, with Eiffel Towers on them.
As you can see, it can be hard for a tourist even to find a place to put a new lock.
Tourism authorities are well aware that attacking the locks would look unromantic, but they're also fed up.
There are just too many locks now. Look how many fewer there were in 2011!
