Let me paint a little picture: you are in Paris, the quintessential city of love, or warmed by Guinness and walking along the River Liffey in Dublin, or enjoying the Christmas markets of Cologne, and you come across a bridge where others have affixed countless locks. Immediately, your heart is aflutter. We should absolutely do this (!!), you both say at the same time, confirming, of course, that you're absolutely of the same mind and totally meant to be together. Oh, and look how convenient! There's a man selling locks right by the side of the bridge. It's kismit!
Hold up. Slowly take a step back from the railing. First, that guy selling locks? If he's not the dude pickpocketing tourists outside the Louvre, he's at least a padlock privateer, separating starry-eyed lovebirds from their hard-earned euros by peddling overpriced wares. Not to rehash Econ 101 or anything, but by positioning himself where the need is greatest, he can now gouge you for a child-sized lock that will most likely fall off the bridge during the next Parisian rainstorm; callously washing your commitment into the river with it.