Depicted we have a strong robust padlock juxtaposed against the prettiest of ribbons atop. The masculine set against the feminine perhaps?

Certain we can feel sure that the padlock denotes a carefully protected fortress of a heart. Impenetrable; forged in cold, hard, heavy metal.

And we can all understand an individual locking away their tender heart for fear of pain and damage, can we not?
Self protection and survival. But ultimately also sluggish and empty.

The relief is provided through the phrasing, you have the key.
The seal owner is giving a glimmer of hope that the recipient of their note may be the one to unlock the hefty iron shackle, to allow the warmth and joy and romance and hot damned sex to come flooding out right in their direction.

And frankly who could resist such an enticement?