What mingles with the river and ocean in "Love's Philosophy"?

According to Percy Bysshe Shelley, the fountains mingle with the rivers; the rivers themselves mingle with the ocean.
Shelley is explaining that nothing is pure and solitary. Everything mixes with something else. Everything joins together. He speaks of the mountains kissing the heavens and the waves embracing one another. Then he addresses the person to whom he's writing and says that they should do the same. He asks, "Why not I with thine?"
In the second part of the poem, he asks what the goodness of the world is worth if he isn't with the person to whom he's speaking. Shelley sees all these things as good things, but they lose their value if he doesn't get to kiss the person he cares for. He's suggesting that the two of them follow divine law and embrace one another like the rest of the things in the natural world.