Can you explain what Keats means in these lines from "Ode to a Nightingale"?Now more than ever seems it rich to die, To cease upon the midnight...

John Keats was dying from tuberculosis when he wrote this
poem, indeed, when he wrote most of his poetry. The disease was called consumption in
Keats's time, and that was a good name for it. The lungs are literally consumed by the
disease until the sick person is left to suffocate to death. In the lines preceding the
ones you have quoted, the poet says:

for many a
time
 
  I have been half in love with easeful
Death,
 
Call'd him soft names in many a musèd
rhyme,
 

  To take into the
air my quiet breath;

So he has been
ready to die for some time. But now, with the nightingale's song in the air, "Now more
than ever seems it rich to die." What a beautiful way to end such ugly
suffering.