"Trees" opens with a candid admission of the poem's insufficiency: the speaker is a poet who, while clearly dedicated to the act of writing poetry, feels that no "poem" could ever live up to the "love[liness]" of a tree. In two short lines, then, the poem sets up a contrast between human creativity and the natural world. For all the efforts of the human imagination, the simple beauty and majesty of a tree will always prove superior.
In theory, the poem could end here! But it doesn't, of course, hinting that there may yet be a purpose to writing poetry after all—even if it can't hope to compare to the natural world. And though the speaker has begun with a confession of inferiority, the poem still strives to construct its own form of "love[liness]." There are a number of ways in which the poem does this, even in this seemingly simple opening couplet.
First, there's a lot of sound patterning at work here, through both alliteration and consonance:
I think that I shall never see
A poem lovely as a tree.
It's a subtle effect, but because the lines are so short and the vocabulary so simple, the /th/ (voiced and unvoiced here) and /l/ sounds shine through. Both are gentle and soft, perhaps embodying how the speaker feels humbled before the divine presence of a tree.
Second, the meter is technically perfect. If readers scan "poem" as having two distinct syllables (as the speaker seems to intend), then both lines conform exactly to the poem's iambic tetrameter scheme (meaning each line has five poetic feet in a da-DUM rhythm):
I think | that I | shall nev- | er see
A po- | em love- | ly as | a tree.
The steady meter here creates a sense of simplicity, grace, and balance. This is intended to make the reader feel that they are in the presence of the perfection. Every syllable is placed with specific intent, hinting at God's design for the world.
Finally, it's also worth noting how the pure, simple rhyme of "see" and "tree" also suggests harmony and simple beauty. The poem thus simultaneously stresses its own inevitable failure and takes on the challenge of creating something that is "lovely"—even if it won't end up as lovely as a tree.