"Reason" was composed by Yoon Jong Shin and written by Park Jin-Young, with English lyrics by Park Jin-Young and An Young-Min. The song was performed by OST, a group created specifically for the drama's soundtrack. The ballad's soaring vocals, poignant lyrics, and sweeping orchestral arrangement have made it a timeless classic in K-pop.
The original song, "Autumn Leaves" (), was composed by Lee Min Hyung and Choi In Kyung, with lyrics by Park Wan Young. Ostsong's rendition, "Autumn in My Heart," offers a fresh perspective on the classic, with English lyrics that convey the same emotions as the original. reason ost autumn in my heart english lyrics