How does the poem bring out the elusive nature of happiness in human existence?

The poem is all about describing and distinguishing between happiness that exist in abstract form and real form which is from inside, the poet talks about the pleasure that he was getting from feeling nature and listening to bird’s song and pain when he was thinking about obstacles in human life. The mortality of bird’s song had been conversely compared with the immortality of human life which ends up with elusive nature of happiness as thinking about this poet could hardly find he was happy. Poet was praising the real happiness but elusive in nature which he was getting while listening to the song of the nightingale.