"Love's Philosophy"

The fountains mingle with the river,

And the rivers with the ocean;

The winds of heaven mix forever

With a sweet emotion;

Nothing in the world is single;

All things by a law divine

In another's being mingle--

Why not I with thine?

See, the mountains kiss high heaven,

And the waves clasp one another;

No sister flower could be forgiven

If it disdained its brother;

And the sunlight clasps the earth,

And the moonbeams kiss the sea;--

What are all these kissings worth,

If thou kiss not me?

This poem is saying that there are small steps that lead to the big picture, for example the fountains, the river, and the ocean, they all connect in some way with some sort of emotion. The small entities, the fountains and rivers, all come together with the big whole,the ocean. Every little thing you do can make a big impact on other people, not just yourself. 

When Shelley says "nothing in the world is single", everything is not alone. In other words, the things in the world are surrounded by things in god's power, or the divine things. Then at the end he asks "In another's being mingle-- Why not I with thine?", he wants to know where his love is if nothing in the world is single and everything is mingling with each other. 

The last stanza talks about how nature can be a symbol of love. He describes the things that can happen in a relationship such as, forgiveness, kissing, and clasping. At the end of the stanza he asks "what are these kissings worth, if thou kiss not me". He is trying to say that if the kisses of nature mean something special, then why doesn't get kissed. This goes back to the first stanza where he asks where his loved one was. He is also describing love's philosophy by using nature's love. Nature is used symbolize human characteristics, not just love. So when he talks about "the moonbeams kiss the sea" and "the winds of heaven mix forever", he is using them to symbolize the characteristics of a human relationship.