The song was written by Lopez and Cory Rooney for the Adam Shankman-directed romantic comedy The Wedding Planner (2001).
Shankman, however, felt that the song had too much of a Latin-influence to be featured in the film, and it was instead included on Lopez's second studio album, J.Lo (2001).
"Ain't It Funny" was released as the third single from the album on June 20, 2001, by Epic Records.
"Ain't It Funny" was a commercial success, peaking within the top ten in multiple countries.
Lyrically, Lopez sings about "creating the perfect romance in your mind, then facing reality when Mr. Right is less than ideal".