Tommy Mottola was so enthusiastic about the song that he contributed to its melody. He then worked with Lopez for an hour to complete the song. After he conceived the idea for 'Ain't It Funny', Rooney returned to his home and 'roughed out the track' and composed the song's chorus. According to Rooney, 'all those words and melodies just started coming' to him while watching the scene. The scene where Lopez's character is in the backseat of a car listening to Matthew McConaughey and Bridgette Wilson's characters talk about the wedding made Rooney think of the line 'ain't it funny how some feelings you just can't deny and you can't move on even though you try'. The two 'pulled different thoughts and feelings' from the film's scenes. While watching a rough cut of The Wedding Planner (2001) in Canada, Jennifer Lopez and Cory Rooney brainstormed ideas for its accompanying soundtrack. The song shares the same title with Lopez's 2002 song ' Ain't It Funny (Murder Remix)', which is a completely different song. Lopez herself was also featured in all three ads. The song was used as background music in three television advertisements in Japan to promote the 2001 Subaru Legacy for a special campaign which was known as Three Keys Legacy. 'Ain't It Funny' was released as the third single from the album on June 20, 2001, by Epic Records. 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).

It was written by Lopez and Cory Rooney for the Adam Shankman-directed romantic comedy The Wedding Planner (2001). ' Ain't It Funny' is a song by American singer Jennifer Lopez.

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