‘Love’s Philosophy’ is a poem by the second-generation Romantic poet Percy Bysshe Shelley (1792-1822). The poem was published in December 1819 and is one of Shelley’s most accessible short poems. Nevertheless, a few words of analysis may help to illuminate the poem’s meaning. First, though, here’s the text of the poem.
Love’s Philosophy
The fountains mingle with the river
And the rivers with the ocean,
The winds of heaven mix for ever
With a sweet emotion;
Nothing in the world is single;
All things by a law divine
In one spirit meet and mingle.
Why not I with thine?—
See the mountains kiss high heaven
And the waves clasp one another;
No sister-flower would be forgiven
If it disdained its brother;
And the sunlight clasps the earth
And the moonbeams kiss the sea:
What is all this sweet work worth
If thou kiss not me?
‘Love’s Philosophy’ comprises just two stanzas, each composed of eight lines, rhymed ababcdcd. The metre is trochaic tetrameter and trimeter: the metre of song. In many ways, ‘Love’s Philosophy’ is a ‘philosophy’ or argument set as a song, and indeed the poem has been set to music on a number of occasions: Roger Quilter set it to music in 1905. ‘Trochaic’ metre is when a stressed syllable is followed by an unstressed one, e.g. ‘NO-thing’ or ‘MOON-beams’. So in the line, ‘See the mountains kiss high heaven’, we might ‘scan’ the line in poetic terms as ‘SEE the MOUNT-ains KISS high HEAV-en’, where the capitalised syllables are the stressed ones. (A number of the lines begin 
with an extra unstressed foot, e.g. ‘The FOUNT-ains MIN-gle WITH the RIV-er’, but the ‘ground plan’ of the poem is still trochaic. This lends the poem – and the poet – a forceful, decisive tone, which is appropriate, since ‘Love’s Philosophy’ is about Shelley trying to seduce a woman to go to bed with him.
Anyway, that’s the technical aspects out the way. What is Shelley actually saying in ‘Love’s Philosophy’? What is the argument of this poem? In essence, it’s a seductive poem, a poem of seduction, an attempt to persuade the (female) addressee to join with the (male) poet in an act of union, shall we say.
Shelley makes his argument by drawing parallels with other areas of nature. The fountains mingle with the river, and the river mingles with the ocean: they are happy to join themselves with something similar to them (they’re all composed of water) and yet distinct. The different winds mingle together in the sky or heavens, and this produces a ‘sweet emotion’. Indeed, it’s not only natural but decreed by God that nothing should be single, and that everything should ‘meet and mingle’ with something else. This happens by divine as well as natural law.
If this is so, in nature, Shelley pleads, then why won’t the addressee mingle with him? Note how he isn’t actually talking strictly about physical union here, but a spiritual joining: in the last line of the first stanza, he’s lamenting the fact that he is not allowed to mingle with the addressee’s ‘spirit’ (‘thine’).
Shelley continues this line of argument in the second stanza of ‘Love’s Philosophy’. The mountains are so tall they seem to ‘kiss’ the skies or ‘high heaven’ (note his use of ‘heaven’ here, combining the awe-inspiring or sublime majesty of nature with the divine once again), the waves of the sea seem to ‘clasp’ each other, and so on. (His vision of two flowers as being childlike siblings is like an older boy asking a young girl out with him, telling her that she shouldn’t hang out with her male sibling all the time but should spend some time with other boys doing … more grown-up things.) But none of this is worth anything, Shelley concludes, if the woman will not kiss him!
Note how he begins by describing how the mountains ‘kiss’ the heaven, and ends by suggesting the idea of the woman kissing him. The repetition of ‘kiss’ in these two contexts reinforces the idea that it would be perfectly natural for her to kiss him: look, even the mountains are doing it to the sky! Similarly, the repetition of ‘clasp’ in the middle lines of this stanza brings together the disparate aspects of the poem. Rhetorically, Shelley is using these echoes to hammer home the idea that everything in nature follows the same law, and what’s more, it’s a law that is created by some higher power (we should perhaps be wary of ascribing this to the Christian God, because Shelley was an atheist who even got thrown out of the University of Oxford for co-authoring a pamphlet called The Necessity of Atheism; but like the other Romantics, he was possessed of a pantheistic belief in the divinity of nature).
‘Love’s Philosophy’ is, indeed, a Romantic poem as well as a romantic one: it is a poem which gives the divine stamp to the notion of ‘free love’, the idea being to pursue our emotions and attractions of ‘spirit’ at any cost. But Shelley expresses this idea using familiar imagery and keeps the argument plain and accessible. As chat-up lines go, it’s expressed better than most.