I think what's going on here is a musical incarnation of the ancient artistic principle of repetition and variation. Art relies on both dynamics: repetition gives the pleasure of recognition and familiarity. We hear something and then hear it again, and we have a sense of inner joy from the recognition of it. Too much recognition and repetition can be boring, though, and so art also introduces the element of variation: a contrasting section in a related key, for instance, or just a different tune to break the monotony of a single melody.
Today's primary example is the great Beatles' song Yesterday, by John Lennon and Paul McCartney. Again, like we had with "Moon River," we have a very condensed lyric with no repetition or additional stanzas after the first four have been exposed:
Look at the simplicity of the form: AABA. All three "A" sections are exactly the same musically. Each starts and ends with the same word in they lyric, "yesterday" or "suddenly". Each starts on the second tone of the scale, an unusual, dissonant tone on which to begin, giving a sense of being unsettled right from the start. We don't know what's going on, only that something is different now from before, causing the plaintive tune (in F major, an interesting choice, in a song where one might expect a minor key) and the mournful cello accompaniment. The contrasting "B" section lets us in on the singers trouble: of course, it's a girl. It probably wasn't her fault, the final "A" section indicates that the singer thought love was a game, so she was probably smart to get out, and everyone is the wiser, and we have this lovely perfect little song to show for our new experience.
All my troubles seemed so far away
Now it looks as though they're here to stay.
O I believe in yesterday.
I'm not half the man I used to be.
There's a shadow hanging over me.
O yesterday came suddenly.
Why she had to go I don't know, she wouldn't say.
I said something wrong, now I long for yesterday.
Love was such an easy game to play
Now I need a place to hide away,
O I believe in yesterday.
(c) 1965 Apple Corps Ltd.
Writers of popular songs, including hymns, have long known that the formula of repetition and variation (surprise) is one that works with audiences, whether in the concert hall or church assemblies. So many popular songs depend on "refrains," repeated sections that recur with some kind of musical or lyrical hook that make us want them to come back again and again. They're made for our memories, with built-in variations to help bear the weight of repetition. Think about any of your favorite church hymns. Following the lead of Luther and Wesley, look at their structure. Take the popular Welsh tune HYFRYDOL (Alleluia, Sing to Jesus). The first and second musical lines of every stanza are identical, the last two are different but related (similarly, DIX (For the Beauty of the Earth) and EIN FESTE BURG (A MIghty Fortress). Or the tune ODE TO JOY, by Beethoven: of the four musical lines, lines 2 and 4 are identical, line 1 is nearly identical, and 3 provides contrast from the dominant key (AA'BA). Or ELLACOMBE (The Day of Resurrection) or LLANFAIR (Hail the Day that Sees Him Rise) - same basic 4-line structure. Even melodies with slightly more complex form rely on imitation in the melody when they don't exactly duplicate it, like NICAEA (”Holy, Holy, Holy,” ABAC) and MENDELSSOHN (”Hark, the Herald Angels,” AA'BBCC’). Many others use refrains and other techniques, like inner repetition, to achieve the familiarity that singers and listeners desire.
Another perfect little song, “Edelweiss,” by Rodgers and Hammerstein from the musical The Sound of Music, demonstrates some of these techniques as well, as it imitates the folk melody and the wisdom of repetitive structure and a condensed lyric:
(A) Edelweiss, edelweiss, every morning you greet me.
(A') Small and white, clean and bright, you look happy to meet me.
(B) Blossom of snow may you bloom and grow, bloom and grow forever.
(A') Edelweiss, edelweiss, bless my homeland forever.
The simple elegance of this lyric, represented here in its entirety, represents well the small flower that the song honors, and the human virtues of honor and solidarity for