Singer Yoon Sang revealed why he offered IU a sad ballad song. 

During the latest filming of 'Yoo Hee Yeol's Sketchbook', Yoon Sang shared the story behind how he gave IU "The Story Only I Didn't Know", which he composed. 

He said, "Before IU became the 'nation's little sister' with her song 'Good Day', we met on a variety show. After seeing her sad face and how she was unable to get along with the other guests on show, I composed 'The Story Only I Didn't Know'."

Yoon Sang is also known to have produced songs for BoA and TVXQ among others. When asked if there was an idol star he would like to write a song for, he answered jokingly, "I can't reveal who it is because of business, but if there are any requests, I'll do my best."

SEE ALSO: "It oozes and swells... the skin gets permanently damaged," IU reveals she suffers from Skeeter Syndrome

Check out IU performing "The Story Only I Didn't Know" below!