This song is the first of Lennon's compositions that may be characterized as "lyrical impressionism," the most glaring / salient example of which is "I Am the Walrus."
From WikiPedia: the "musical impressionism (of the 19th century) focused on suggestion and atmosphere rather than strong emotion or the depiction of a story."
It’s not clear that Lennon consciously set out to create a new "artistic movement" in popular/rock music – in other words, that one day he said to himself, “I think I’ll write some songs that superficially appear to be nonsensical and meaningless – which, in fact, HAVE NO concrete meaning - but which, nonetheless, strongly suggest some kind of meaning … conveying an interpretation and / or feeling subliminally.”
But, as if to acknowledge that he was “up to something,” "I Am the Walrus" itself refers to another example: "Lucy in the Sky with Diamonds;" and the last example by Lennon, "Come Together," contains the phrase, "he has walrus gumboot."
Precisely why Lennon wrote such songs is open to speculation. Probably he wrote them because he could (and not many good songwriters can), but Lennon also seemed to enjoy being a bit inscrutable at times, as if his ability to mystify the masses were evidence of his intellectual superiority (which perhaps it was).
Much of the mood of the song is created by the refrain, "you can talk to me - if you’re lonely, you can talk to me!" though like other examples of impressionistic lyrics, the song stirs one's feeling with aphorisms (little gems of truth) such as "some kind of innocence is measured out in years."
The song is also an example of the wonderful interplay between Lennon and McCartney, two very different (to a certain degree, diametrically opposed) personalities, salient examples of which are "A Day in the Life" and "I've Got a Feeling."
In this song, specifically, McCartney "corrupts" the ambiance of Lennon's avant-garde lyrics with his clowning at the end (which Lennon replies to, with his own clowning), turning it into a fun, rollicking, quintessentially rock-n-roll song - superbly illustrated in cartoon version in "Yellow Submarine."
From WikiPedia: "Geoff Emerick, the Beatles' engineer, would subsequently claim this was the last song the band recorded that featured a team dynamic, with enthusiasm from every member."
Sheryl Crow's "If It Makes You Happy" is yet another example of "lyrical impressionism."
