That rabbit hole wasn't as deep as I expected. Here's an excerpt from an article I found here
"...the love lock tradition didn’t even start in Paris — it started in a town called Vrnjačka Banja in Serbia. There, shortly before the First World War, a young man and woman fell in love, and would meet every night at the Most Ljubavi bridge in town. But the man went into the military, and while abroad, he met and fell in love with someone else. The young woman died of heartbreak, and superstitious local women began going to the bridge, writing the names of themselves and their lovers on padlocks, and locking them to the bridge, in the hope that it would bind their paramours to home."