[1][3] The song was written by Madison Beer, Leroy Clampitt, Jaramye Daniels, Jeremy Dussolliet, Elizabeth Lowell Boland, and Tim Sommers.

[4] In terms of music notation, "Selfish" was composed using 44 common time in the key of A major with a slow tempo of 75 beats per minute.

Billboard magazine praised Beer's vocal performance on the song, writing that the singer "sounds poised [on "Selfish"], committed to the approach and confident in her capabilities as a vocalist".

[3] "Selfish" was accompanied by a music video co-directed by Madison Beer and Jason Lester that was released the same day as the single.

[8] The video features the artist lying down on an illuminated platform, she progresses throughout the song with more powerful runs that weren't originally present in the track.