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.