The song was written by group members Hyerim, Sunmi, and Yubin, along with songwriter Hong Ji-sang who also handled the track's production.

To promote the single, Wonder Girls performed the song on various South Korean music programs, winning awards on The Show, M!

Unlike Reboot, which had hired instrumentalists, the members actually played their instruments on the tracks, completing their transition from a dance-oriented girl group to a real band.

[2][3] "Why So Lonely" was described as a slow tempo, reggae-pop dance song with "soft guitar riffs and steel drum rhythm".

[5][6] "Beautiful Boy" was released exclusively on limited edition vinyl at the 6th Seoul Record & CD Fair on June 18, selling all 500 available copies in less than two hours.

[9] Jeff Benjamin of Fuse noted the song's 1970s-inspired rock band concept and reggae-rock sound, and said "the surging guitar riffs mixed with Wailers- and Sublime-like ska beats give the track a classic-rock vibe that the ladies successfully pull off".

[11] Billboard named "Why So Lonely" the second best K-Pop song of 2016, calling the band "throwback queens" and stating they were able to conquer "another musical era" while "touching listeners' emotions.

[21] The single was promoted with a series of televised live performances on South Korean music programs, starting with Mnet's M!

[6] Before the single's release, KBS had banned "Sweet & Easy" from broadcast because the lyrics mention a brand name, considered "inappropriate for the national telecast".

[22][23] On July 12, Wonder Girls performed the dance version of "Why So Lonely" for the first time on SBS MTV's The Show, winning the trophy for that week.