One common theme is the idea of a private, yet somewhat public, space for love to develop. The balcony is a place where the characters can be semi - isolated but still part of the outside world.
Frequently, the theme of secrecy can be found. The balcony might be a place where a relationship is secretly nurtured away from prying eyes. It could be a forbidden love situation, and the balcony is their haven. Also, the concept of transformation is present. The characters may transform from being strangers or just acquaintances to being deeply in love through their balcony interactions.
Another theme is often the element of discovery. Characters discover new aspects of each other while on the balcony. For example, they might find out about each other's hobbies or dreams through conversations held there. It's also common to have a theme of longing. Maybe one character is longing for the other to make a move or for their relationship to progress further while on that balcony space. And often, the beauty of nature around the balcony, like the sunset or the flowers, serves as a backdrop to enhance the romantic mood.