When a couple gets married, a padlock on a bridge is a way to symbolize that eternal union.
Engraved locks of eternal love. Baraskit.
..when couples get married they put a padlock on the bridge as a symbol of their commitment to each other.
From one end to the other, nonstop padlocks on this bridge here.
Image courtesy of http://www.baraskit.se/random/archive/170/padlocks_on_bridges.html.