Rain is the one comparison the speaker keeps returning to in "Rainy Day," and it works as a way of describing how love feels. The opening lines link falling raindrops to falling in love, and the speaker says both keep going until the storm has run its course. Love, in this view, isn't something you control. It arrives and carries on by itself.
The middle of the poem admits that love isn't only happy. A rainy day brings "an overflow of sadness," but a calm, pleasant day follows. The speaker seems to accept the pain as part of the deal, and the poem ends on hope rather than hurt.
The mood is quiet, sincere, and a little wistful. The language is simple and the lines are short, which makes the feeling seem unguarded. The speaker insists love will never die, and the final lines repeat that promise by returning to the rain, which only stops when it reaches its end.