This is a bridge in Paris. You hang locks on it with the name of you & your boyfriend/girlfriend/best-friend then throw the key into the river. so even though the friend/relationship may end, you can’t remove the lock. It stays there forever, as relevance to someone once a part of your life.
As long as I’m on the subject of bridges… I love this idea. Because relationships of any kind - real relationships - never end. For good or for ill, they have a lasting impact on us for as long as we live. And (depending on what you believe) long after we live.