I was trying to analyze this song from a music theory perspective, to help internalize why it works, and employ that in my own songwriting. I read some discussions in other forums about this song, but I have a different spin on things, which I wanted to throw out, see if it makes sense as a way to look at song analysis.

The verses in this song are G | B7 | C (or C7?) | A (or A7? or slide B7-Bb7-A7?)
Then chorus (?): G | E | G | E | G | A | G | E
Then interlude (?): G | D | C | G | D | C | G | D | C | F | D

So, the gist of what I read elsewhere breaks down into two ideas:
First, as the song is Key of G, B7 after G works because it is the dom of the relative minor. And while it most strongly resolves to Em, C is an accepted substitute for Em so the B7-C resolution is what's called a "deceptive cadence." And, too, the C is in the key of G, so it feels reassuring as to the key of G, to come back to C. Then the shift, or slide down, to A works because A7 is the double dominant of G? A7 is the dom of D7 which is the dom of G? So the A leads the ear to expect D, but by not going there, the song leaves a tension in the listener that helps drive the song on. And as a double dominant, A is not too far from key of G.

The second explanation I read focused on the chord shifts, and the move by thirds that remain in the major key (C-A, G-E and, later, F-D). The phrase "chromatic mediant relationships" was used. I'm not sure I fully understood this, but it actually fits what I had been thinking when playing the song, is that it works in part because of parallel movements between chords as in these three places.

But what I find missing in these discussions is why the song works in emotional terms, or on a feeling level. I'm frankly not sure I have the answer, but one idea I had was that music can create music by moving from major to minor, or towards a lower key, in a way that can feel melancholy or even depressing, whereas music can move the other way to create energetic emotions, like joy or even anger.

So here we have a song that starts in key of G and then the move the B7 is relatively neutral. In the key of G, this should be B minor, but replacing B major could add an energy, like shifting from a minor to major key. Yet, the B7 is a bit weaker than a full B chord would be at this, and as it is the dominant of Em, it also causes the mind's ear to anticipate an Em, which is a shift from a major to relative minor. So in a way, this chord presents the ear with both a minor to major shift, and with a major to minor shift, which kind of cancel out and we are just "on hold" emotionally. We get back to C, which relaxes us since it fits well with the original G key. Then we move to A, which is unsettling. The verse works by being unsettling, and keeping the tension up for a stronger resolution in the chorus/interlude parts. But, also, the A rather than A minor adds an energy because that's what happens when you substitute a major for minor, you get an emotional charge whereas the expected Am would just be sort of a non-issue. Moreover, A is the dominant of D. And when you have both G and A in a song, the ear may want the song to resolve to D.

D is clockwise in the circle of fifths, and I tend to think of a key change this direction as adding energy similar (but to a lesser degree) than a shift to a higher key like G to A, whereas a counterclockwise shift (e.g., G to C) detracts energy, creates a subdued feel. So, in lay terms, the A at the end makes the chorus feel like a key of G flirting with key of D, or thinking about the idea of a positive shift. In any case, we have D on the mind while we are in the key of G, which is sort of like daydreaming. The B also fits as it is the relative minor of D, so even though its B7, it also fits in the key of D. And, if you look at the progression, you go B - C - __? such that the ear might even anticipate a D after the C, so you can already be anticipating D after the C, and instead you get the dominant of D.

The other thing I see is that A and B are the 4 and 5 of the key of E major. So by playing both an A and B in the verse sequence, we also call to mind the key of E, which is energetically a step up from the key of D. So, we feel the song is in the key of G, but strongly implies a move to D, and perhaps a bit less strongly also implies a move to the key of E.

The chorus then plays with this feeling of shifting to the key of E, by alternating between the chords over and over, like we are offered a choice -- here or there? Then the playing of A -- which is in the key of E, not G, seems to make up our mind we'll go to the key of E after all. However, that last progression A - G - E feels somewhat resolved to E, yet not fully because the A - G progression strongly wants to go to D. So after all this "G or E, G or E" back and forth, we remind the mind's ear that the key of D was an option, so we settle on E with perhaps a bit of second thought.

And at that moment, we retreat strongly back to the key of G, with the 1-5-4-1 progression over and over. At the same time, we are also repeatedly playing the D, which has been only hinted at -- quite strongly -- in the preceding progressions, so there is a strong release every time the D is played. In fact, we keep play 1-5, so that when we end on 1, the ear again wants the D, the 5. So the listener is a bit conflicted. We want the D, but we are also feeling very at home in G with all the 1-5-4's.

The last jump from G to F swings us strongly to the other way, like a pendulum. Instead of flirting with clockwise to the key of D, the G-F now flirts with a move counterclockwise to the key of C, a defusing and perhaps melancholy move, particularly as F is the relative major of D minor, so in a way we are sort of hearing a D minor in place of D major, so there is a major to minor feel. Change can be bad or good. If we change, it might not all be berries and cream or whatever. And after we feel that risk, we then swing the pendulum back with the F-D, which is a form of minor to major change, since D minor is relative minor of F. This swings sort of cancel out and leave is in between C and D, which is right on the key of G, very much at home, and ending on the dominant of D so we are ready for the restarted of the verse with the G chord.

Well, it may seem like I'm "overthinking" it, but I guess the point I'm trying to make is that I look at this or other songs, and I tend to try to see how they work by "anthropomorphizing" the song, and seeing it in terms of a certain key flirting with other keys, perhaps more so than a particular chord change evoking a certain feeling. To me, this song is a key of G flirting with different moves, to D or E, and once even a change to C, but ultimately staying in the key of G. Which fits the lyrics which involve a person who has decided to stay put and just sit where he's at while watching travel / movement (of ships) around him, and thinking of how people want him to change himself, which he refuses to do.

If I'm writing a song in a certain key, I'll generally have an idea where I want to get the tension, like flirting with a key change by steps, or along the circle, or actually making a key change. Same, too, for chorus. Do I want to mellow things out? Kick things up a notch? Depending what I want to feel, I think about what other keys I should move to, or at least flirt with. So to me music composition is largely about the relationship between keys, first and foremost, rather that, say, the relationship between notes or even chords. But I tend to see most song deconstruction focusing on each chord to chord or even note to note change rather than the big picture of keys. But perhaps I have just not read the right books?

Bernie Sanders for President!
good work on your over analysis...the song was written by otis redding & steve cropper...the chord changes are all "soul flavored" harmonic devices that cropper has used for years on many other famous songs...(reddings "been loving you a little too long" comes to mind)

using dom 7 chords in place of diatonic function chords is what gives blues , "soul" and gospel its unique sound and feel .. so its not uncommon to see I - III7 as a first change it has a nice feel..gets your attention..as it is a bit unexpected...the C to A7 / IV - II7 via passing chords is also a nice harmonic treat..again the cropper sound that he uses with taste..

i suggest you research not only the songs..but the songwriters also as this may give you great insight into their method of writing..as many have harmonic devices that make them recognizable to the ear.

play well

I understand Otis didn't know how to play minor chords, hence all the major chords
But boys will be boys and girls have those eyes
that'll cut you to ribbons, sometimes
and all you can do is just wait by the moon
and bleed if it's what she says you ought to do