"Why So Lonely" is a song recorded by South Korean girl group Wonder Girls. It was released as a CD and digital single by JYP Entertainment on July 5, 2016, distributed by KT Music. The song is a reggae-pop dance track written by members Hyelim, Sunmi, and Yubin, with songwriter Hong Ji-sang. The single has two B-sides, "To the Beautiful You" and "Sweet & Easy"; the former was released on limited edition vinyl on June 18, 2016. To promote the single, Wonder Girls performed "Why So Lonely" on South Korean music programs, winning awards on The Show, M! Countdown, Inkigayo, and Music Bank.

The song received favorable reviews from music critics, who praised Wonder Girls for successfully attempting reggae, a new sound for the group. The single was commercially successful in South Korea—the digital download topped the Gaon Digital Chart, while the CD single charted at number three on the Gaon Album Chart.

"Why So Lonely" was written by Wonder Girls members Hyelim, Sunmi, and Yubin, with songwriter Hong Ji-sang. It is the first Wonder Girls single not written by JYP Entertainment producer Park Jin-young, and marks their tenth anniversary as a group. After the release of the group's album Reboot in 2015, Park told them they were ready to write their own single. Yeeun said they tried to write a song that can "really benefit from being played live as a band", because their previous release did not meet their expectations. The reggae genre was chosen after the members voted on various options, and they were also influenced by British singer Lily Allen. 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.

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