Being in love is a wonderful feeling. But how about being on a trip or vacation and your loved one is at home, not knowing how much you miss him/her? Sending an ordinary postcard in the era of internet is so lame, so you have to figure out something that is more sophisticated and classic. If I were you, I would go to this spot, take a picture, upload it on some social network and wait for a response.
Beneath the sign there is a graffiti which memorizes the 2013 march against cuts in the public sector organized by 15O, the Slovenian version of the 15th of May Social Movement in Spain (Movimiento 15-M). You can check what was happening here.
Beside that one, you may find several others around the city with the same style but different meanings. “You never forget your first one” is hiding close to the Reber stairs.