We noticed some people have their padlocks professionally engraved before coming, and others write or scratch onto theirs. Yes, there's a guy on the bridge who will sell you one for 2 euros. Disturbingly, some are combination locks. What does that say about the relationship?
More disturbingly, we discovered this quaint and personal ritual while playing with iPads in the nearby Apple store - I ran a travel app, and asked it "what is good near me?".
Incurable romantics, those iPads!