Can you explain the poem "Theodore The Poet" from Spoon River Anthology?

I regard this poem as more philosophically existential than the others; it does not reflect on a life that was lived and the errors that were made, as the others do, but instead on the meaning of existence.

The narrator addresses himself in second-person, which establishes some objective distance:

As a boy, Theodore, you sat for long hours

On the shore of the turbid Spoon...

He is speaking to himself as a boy, sitting along the banks of Spoon River, which he describes as "turbid," or muddy. The adjective used here also reflects the murkiness of our understanding of life's meaning.

Nevertheless, the young Theodore was intent in figuring out what it all meant:

With deep-set eye staring at the door of the crawfish's burrow,

Waiting for him to appear, pushing ahead,

First his waving antennae, like straws of hay,

And soon his body, colored like soap-stone,

Gemmed with eyes of jet.

His own sensibilities merge with those of the crawfish. He gives the crawfish a "door" to his "burrow" and decides the crawfish is male, referring to it as "him." The textures on the body of the crawfish contrast and are identified with mundane objects: the flimsy "antennae" are like "straws of hay," the "body" is like "soap-stone," which is sturdy at one moment and dissolves the next, and the "eyes" are "gemmed." With these comparisons, the crawfish is at once fragile yet sensate, sturdy yet vulnerable, and precious yet common. One could transfer all these senses to our understanding of what it means to be human.

He shifts from this contemplation of the crustacean's body to a contemplation of the world as a crawfish would experience it:

And you wondered in a trance of thought

What he knew, what he desired, and why he lived at all.

Why does the crawfish matter? What purpose does it serve? What purpose, we could ask, do any of us serve?

This reverie then incorporates the human world:

But later your vision watched for men and women

Hiding in burrows of fate amid great cities,

Looking for the souls of them to come out,

So that you could see

How they lived, and for what,

And why they kept crawling so busily

Along the sandy way where water fails

As the summer wanes.

The "burrow" into which the crawfish crawls is not much different from that in which men and women "[hide]." What is interesting is that Theodore gives the crawfish a "door" while the men and women are "in burrows of fate," which gives them less control over how they may exit or enter. Theodore suggests, then, that human lives are predetermined. He gives them "souls," which seem to have the crawfish's ability to move more freely ("the souls of them to come out"). 

Theodore's "vision" imagined how these people lived and for what, in the way that he also imagines why the crawfish lives, and for what. He reverts them to the state of the crawfish: they do not walk, but rather crawl "along the sandy way," or along a space that is slippery and always receding. In other words, they move feverishly, but for nought. Here, the water does not fall but "fails." This is a slight play on words, and a trick on our understanding of what water should do. Water is usually a life-giving force for both the crawfish and people, but is absent here ("fails").

All of this occurs "[a]s the summer wanes." "Summer" is generally associated with the midst of one's life. The men and women crawled "so busy / Along the sandy way" during the summer of their lives, though the season "[waned]," indicating the loss of time and the triviality of their actions.