Why did Lennon and McCartney write a song together ?
After Lennon 's death , McCartney again attempted to change the order to McCartney – Lennon for songs that were solely or predominantly written by him , such as " Yesterday , " but Ono would not allow it , saying she felt this broke an agreement that the two had made while Lennon was still alive , and the surviving Beatle argued that such an agreement never existed .