The theme of this poem is love and the uncertainty and difficulty that comes with love. In the poem, the author is asking a series of rhetorical questions about the ways in which love can seem impossible to understand.
The author focuses mainly on the feeling of uncertainty that comes with love. He talks about how hearts can change as easily as the weather, implying that love can be fickle and changing and therefore uncertain and difficult to understand. He talks about how someone's emotions can go from lowest to highest and back again, all because of love.
A person who has been in love will recognize these feelings from their own experience -- the highs, the lows, the uncertainty, the confusion. This is the major theme of this poem.
Love and pain
The whole poem show us the love between the poet and his beloved. But that love was from the poet's side only. His beloved didn't love him as much as he loved her. His Love turned to be a painful love and that love started aching because of her .
The whole poem is a memorial . the poet is remembering his hurtful experience in love with his beloved who didn't share him those feelings. It ended and broke the poet's heart.