This is another of Simon's songs that seems to be two songs welded together. The first is a love song, the second is a break-up song.
But perhaps it is one song, just in two parts, like an episode of Law & Order. First, we see the explosive relationship being assembled and the fuse lit... then we see the fallout.
There are two people here-- let's call them Chris and Pat, with Chris being the speaker. Chris is an insecure person, regardless of being in a relationship or not: "Nervous when you got it/ Nervous when it's gone" that it is gone for good... and then nervous of losing it again if it comes back.
The problem starts immediately, with the first line of the song. While most lullabies have four angels guarding a sleeping child, over-protective Chris piles "twelve angles" onto Pat, saying "I'd do anything to keep you safe."
Things move slowly-- "little bit by little bit"-- until they are perfect: "Now you got it, that's it." Instantly, Chris starts to "take [the relationship's] temperature every hour," which probably drives Pat batty. Chris also is needy, telling Pat: "You are the air inside my chest."
Then there is a clatter of hand drums, symbolizing discord. Suddenly, there is a break-up! And we start Part II: The Recrimination.
"You're the one!" accuses Chris. "You broke my heart. You made me cry." Here, Simon is mocking the pop-song convention of blaming the other party.
But some part of Chris is rational, after all. When piling on angles, Chris muses: "Maybe that's a waste of angles, I don't know." This part of Chris' mind, capable of analyzing and even debating against its own thoughts, comes back into play. This part asserts itself through a subconscious "dream." Now, Chris is able to put the capacity for anticipating others' needs into use, now, to see another's point of view: "But when I hear it from the other side/ It's a completely different song/ I'm the one who made you cry/ I'm the one who's wrong." [emphasis mine].
Then Simon gives us his moral of the story. Change is constant in "nature," he says, citing the amorphous "shapeless shapes" of "clouds and waves and flame." But "human expectation," unreasonably, "is that love remains the same."
So Chris does the obvious thing: "Blame, blame, blame." Whose fault is it? Chris, for being smothering and clingy? Pat, for not proving some sort of reassurance, or for enabling Chris' neediness to persist past the breaking point?
Yes, and yes-- it's everyone's fault, Simon concludes: "We're the one."
Next Song: The Teacher