This song (approximately, at least) marked the transition from the old twee pop Beatles to the much more interesting act that they became.
Although “Lennon and McCartney” appear on the credits to songs written by only one of them, this was a genuine co-composition. According to Wikipedia, this is McCartney wrote most of it before enlisting Lennon to help finish it off, but Macca credits George Harrison with the idea to waltz-time the end of the “Life is very short” section (where it goes “and fighting my friend”).
This is when the song-writing became a little more sophisticated and, for me, way more interesting. (I’ll do a piece on Happiness is a Warm Gun at some point - that’s crazily all over the place - in a good way).