When I was in Würzburg, I saw what has become a very common sight on bridges in Europe:
I personally think it's very childish. It kind of reminds me of the whole friendship necklace thing. Sure, that's cool...when you're 8. Plus, this just looks ugly and it damages bridges. It reminds me of people who carve their initials into trees. I loathe that even more; why damage/weaken a perfectly good tree?! UGH! It's a very selfish thing to do, and again, it's childish.