In Edgar Allan Poe's poem, as noted above, perception is everything. Unless the author has elucidated a personal response to the poem, as readers, we are simply left to wonder about his meaning. Each of us brings our own perception to the poem based upon our personal experiences and reactions to the poet's work.
In "Alone," we are confronted with the sense that the poet has felt differently than those around him...from the time he was born.
From childhood's hour I have not been
As others were; I have not seen
As others saw; I could not bring
My passions from a common spring.
Poe notes that he has been different and felt differently. Things that he feels strongly ("passionately") about are not the same as the experiences of others, noted here as "a common spring." With these people, he has nothing in common. He is "alone." In sorrow he has not been the same, has been unable to draw joy from life as others, and he has been alone in love.
There is a noticeable shift, when he speaks of the "dawn of a stormy life:"
Then--in my childhood, in the dawn
Of a most stormy life--was drawn
From every depth of good and ill
The mystery which binds me still...
From early in his life, it has been stormy—difficult and full of upheaval. He does not understand, for he feels enveloped in a mystery that confronted him when he was young and still has not revealed the truth of itself to him.
Poe makes note of nature passing by, something gently and other times in a "torrent" or like "lightning." Regardless of the guise in which it presents itself, the speaker makes note of the blue sky, which should be synonymous with heaven—gentle, calming and beautiful. However, for Poe, the cloud on the blue backdrop is anything but heavenly.
And the cloud that took the form
(When the rest of Heaven was blue)
Of a demon in my view.
The cloud that he perceives once again separates him from what most people see in a could—and this is the basis of his concern, the source of his sense of alienation. Poe sees not something lovely in the shape of a cloud moving across heavens, but a demon instead. Poe makes note of the image and is aware that he is alone in how he sees and experiences the world.