The speaker admits that he has no idea how the world came to be filled with "dappled things." He can offer no explanation but can only describe and admire. Some religious thinkers would say that nature must be beautiful because it was created by God. Hopkins says that God is praise-worthy because He created such a mysterious and beautiful world. Maybe there's not a huge distinction between the two views, except one of attitude. Hopkins seems to have an appreciation of natural diversity for its own sake, in all things great and small, and regardless of their relation to human ends.
The poem is an expression of naïve and childish innocence that could not possibly be sustained.