Nice post, Lemon[H]ead!
Sandburg could get pretty yeasty, stirring up love's lost yearnings, that's what made him so great. It shows how the heart of a poet works.
This poem is a pastiche of memories about a woman who got away, and a man, lovesick, asking that never-ending question, "WHY?"
She had her own self-determinism, but was she ever aware of how dear she was to the speaker?
Perhaps this is the point of it...do we ever know how much we are regarded by others?
Or maybe...he regrets his foolishness in his hesitation...and now she is gone.
Either way, a powerful evocation of love's labors lost.