The song was written by Jaehyun, Dewain Whitmore Jr., Ellie Suh, Micah Premnath, and producer Tido Nguyen.

It would also be the fourth single featured on NCT Lab, a special season of the digital music project SM Station.

[4] "Forever Only" was written by Jaehyun, Dewain Whitmore Jr., Ellie Suh, Micah Premnath, and Tido Nguyen, who also produced the song.

[10] Jaehyun's vocal performance was described as "alluring", "deep" and "lush" by Divyansha Dongre of Rolling Stone India.

[11] Jaehyun said that after hearing the "bright" melody of the song for the first time he "felt lonely inside of that kind of mood", so he decided to counter the "positive" sound with lyrics centered around the theme of loneliness.

[17] Divyansha Dongre of Rolling Stone India highlighted the "wealth of ad-libs" and "dreamy falsetto performance", saying it ties "the sonic elements together [of the song] beautifully whilst simultaneously adding depth and dimension to the track".

[22] It features Jaehyun singing and playing the guitar, lounging in a room surrounded by photographs, and riding a motorcycle, all in the span of a day.

[7] Dongre said the music video employs "a somber color palette and hazy filters [that create] a gloomy atmosphere".