Though nature loves her, the final two lines of the poem seem to imply that something is preventing her from returning the joy. Her “silver mists” are her tears, and they could be read as tears of joy or happiness at being in nature, but there is a longing here, perhaps because it is winter and she longs for summer, but maybe because she does not understand why nature loves her.
“Wherefore” is one of those words that you see a lot in poetry (especially bad poetry) but though I’ve seen this word a million times I decided to look it up in the OED because since Emily doesn’t write bad poetry, then perhaps she is using this word very much on purpose and so I should really know what it means. The main definition of “Wherefore” is “for what purpose or end?” (OED) and thus embedded in the word is a ‘why’ question. In the second to final line of the poem she is asking ‘for what purpose’ do my eyes fill with tears?’ and then repeats the question by asking the “Summer’s Day” directly why her eyes are filled with tears even though nature loves her and comes alive when she is near.
And I get the sense that she is having a difficult time feeling joy even though she knows those around her feel joy at her company. In other words, she seems to be describing a depression in which even though a person knows they are loved, they feel cut off somehow, unable to connect emotionally to the world around them. The final line where she addresses a “Summer’s Day” (in the apostrophe), there is the sense that she is almost begging for an answer. In the previous lines the “Brooks laugh louder” and the “Breezes madder play” and thus there is a rising emotional quality to the scene as if her mind is filled with competing emotions that are almost overwhelming yet she is unable to return the emotions back to the “Brooks” and the “Breezes” and feels isolated even when she is surrounded by love and joy. In fact there is almost a manic feel to this final stanza with “louder” and “madder” as if she is describing her mind’s turmoil – she can’t relax, she can’t just accept the love that is being shown her and so she can only question it which, she knows, is sad. She wants to feel that joy, but somehow she can’t.
Emily is longing for a “Summer’s Day” in her heart and in her mind. Her season is a season of winter that longs for a “Summer’s Day”, but the cold and winter storms prevent her from feeling the warmth that is otherwise so freely given to her. She knows she is loved, but she finds it difficult to actually feel it.