While the poem itself is reflecting about early childhood, for me this excerpt is a metaphor of something "which having been must ever be".  And that is the beauty of poetry. We take the words of poetry and lay our experiences and emotions on top of them and they reflect back that which our hearts yearn to say or to feel.

On Intimations of Immortality from Recollections of Early Childhood

What though the radiance which was once so bright
Be now for ever taken from my sight,
     Though nothing can bring back the hour
Of splendour in the grass, of glory in the flower;
            We will grieve not, rather find
            Strength in what remains behind;
            In the primal sympathy
            Which having been must ever be;
            In the soothing thoughts that spring
            Out of human suffering;
            In the faith that looks through death,
In years that bring the philosophic mind.
And O, ye Fountains, Meadows, Hills, and Groves,
Forebode not any severing of our loves!
Yet in my heart of hearts I feel your might;
I only have relinquished one delight
To live beneath your more habitual sway. 

I love the Brooks which down their channels fret,
Even more than when I tripped lightly as they;
The innocent brightness of a new-born Day
                Is lovely yet;
The Clouds that gather round the setting sun
Do take a sober colouring from an eye
That hath kept watch o'er man's mortality;
Another race hath been, and other palms are won. 

Thanks to the human heart by which we live,
Thanks to its tenderness, its joys, and fears,
To me the meanest flower that blows can give
Thoughts that do often lie too deep for tear