In an apostrophe to a frozen tree, the speaker tells it that it’s “too happy, happy”—that is, that it’s so lucky. That’s because its frozen “branches ne’er remember / Their green felicity”: they can’t recall what it was like to be lush with leaves. This apostrophe stresses the distinction between a tree’s experience and a person’s: when a person loses something, the speaker laments, they remember all too well what it was like to have it.
Poetically speaking, this apostrophe is pretty novel. Rather than conventionally personifying the tree—say, by presenting it shivering in the winter cold and thus making it into a fellow-sufferer, a symbol of human feeling—the speaker insists that a tree’s experience is simply different from a person’s (and enviably so). This speaker doesn’t make the old Romantic move of turning to nature for consolation and wisdom. Rather, they reach out imaginatively to nature only to discover an alien “sweet forgetting” they only wish they could share.
An apostrophe to the “happy happy Brook” works similarly, making the brook seem remote and strange even as the speaker talks to it directly. As blithely unaware of winter as the tree is, the brook “never never pet[s]” (that is, fusses) about the “frozen time.”
These apostrophes might invite readers to consider whether the tree and the brook feel anything at all, ever. If they don’t feel pain in winter, do they feel joy when Apollo (the Greek sun god) shines his “Summer look” down upon them again? Or is their happiness, their good luck, simply in a uniformly “numbed sense,” an inability to feel anything? To this dejected speaker, nature’s numbness might sound like a pretty good deal compared to the pain of grief.
Rather than bringing nature consolingly closer to the speaker, then, the poem’s apostrophes emphasize the distance between feeling humanity and insensible nature.