Some of it is just growing up.... "Shades of the prison-house begin to close/Upon the growing boy."
Two stanzas from William Wordsworth, "Intimations of Immortality from Recollections of Early Childhood"
There was a time when meadow, grove, and stream,
The earth, and every common sight,
To me did seem
Apparelled in celestial light,
The glory and the freshness of a dream.
It is not now as it hath been of yore;--
Turn wheresoe'er I may,
By night or day,
The things which I have seen I now can see no more....
Heaven lies about us in our infancy!
Shades of the prison-house begin to close
Upon the growing Boy,
But He beholds the light, and whence it flows,
He sees it in his joy;
The Youth, who daily farther from the east
Must travel, still is Nature's Priest,
And by the vision splendid
Is on his way attended;
At length the Man perceives it die away,
And fade into the light of common day.