This, according to the speaker, is the reason why humans can't help but be sad. We're always thinking about what happened in the past and what's coming our way. We can't just feel the pure joy of the moment. It's kind of the tragic irony of this poem, too. The more the poet tries to live in this instant, to communicate the power of the bird's song, the more he realizes he'll never be able to. He's human, and so he's left to feel bad about what he can't have, to "pine for what is not."