Life is very short and there's no time
For fussing and fighting, my friend
I have always thought that it's a crime
So I will ask you once again
The lyrics and music of this song bears the distinctive characteristics of both Lennon and McCartney. McCartney's verses are light and optimistic, carried on an uplifting, simple three-chord sequence in D major.
John's middle eights by comparison are cynical and impatient, "there's no time...", and in them the song switches to a drooping B minor that was inspired by German waltzes (a style in which John would also write Girl).
John said in Anthology:
Paul did the first half, I did the middle eight. But you've got Paul writing, 'We can work it out, we can work it out' - real optimistic, y'know, and me impatient: 'Life is very short and there's no time for fussing and fighting, my friend.'
Paul accredits this waltz influence to guitarist George Harrison in Many Years From Now. The band unusually arranged the song in-studio as they were recording it, which meant George had more opportunity to affect the basic creative direction of a song than he did in most traditional Lennon/McCartney numbers.
It was George Harrison's idea to put the middle into waltz time, like a German waltz. That came on the session, it was one of the cases of the arrangement being done on the session.