Which One Direction song starts with the lyric 'You're insecure, don't know what for'?

The One Direction song beginning “You’re insecure, don’t know what for” is “What Makes You Beautiful.”

The song was released as One Direction’s debut single on 11 September 2011 and served as the lead single from Up All Night. Its opening lyric introduces the song’s central idea: reassuring someone who does not recognise the qualities that make them attractive.

“What Makes You Beautiful” was written by Savan Kotecha, Carl Falk and Rami Yacoub, with Falk and Yacoub producing it. The track combines a guitar-led pop-rock sound with power-pop and teen-pop elements, plus the memorable “na na na” section.

The song became One Direction’s signature early hit. It reached number one on the UK Singles Chart and won the 2012 Brit Award for British Single of the Year. It is sometimes confused with “One Thing” or “Gotta Be You” because all three were early singles, but the quoted opening unmistakably identifies “What Makes You Beautiful.”