That's from a poem by Walt Whitman. So just what does it mean? I don't pretend to know, but I can tell you what it means to me. It means if I put love out into the world I will get it back. Guaranteed.
But that's crazy, you say. What about unrequited love?
If you put love out into the world, you will get it back. It's as simple as that. It may not be the love in the exact form you gave it. It may not be from the person you gave the love to (unrequited). You may not get the love back instantaneously. But you will get the love. Guaranteed.
If you're not getting enough love in your life, the solution is simple: love more. The love will come back to you. Want a lot of love? Love a lot.
It's just. It's simple. It works. Guaranteed.