The title is a neologism referring to the "realization that each random passerby is living a life as vivid and complex as your own".

[4][12] The tracks "Dreamer" and "Innocence and Sadness" were also issued prior to the album and both reached the top 20 in Ireland.

[12] Barney Harsent of The Arts Desk called the album "crowd-pleasing, high-polish pop" with "sparse accompaniments hiding in the wings giving Kennedy's voice the spotlight and space to fire up the feels", but felt "none of it screams raw emotion.

[2] Lauren Murphy of The Irish Times described Sonder as "another record of solid emotional pop songs that run the gamut from heartache to cautious optimism" although neither Kennedy's "musical nor his lyrical themes have developed in any groundbreaking manner since his 2018 debut Without Fear".

While Murphy felt that it will "matter little to Kennedy's established fan base", "it comes across as a missed opportunity to prove his worth as a songwriter with original ideas rather than a one-trick pony".