Welcome joy, and welcome sorrow

John Keats loves the principle of beauty in all things. He expressed this idea in A song of Opposites:

Welcome joy, and welcome sorrow,
    Lethe's weed and Herme's feather,
Come today, and come tomorrow,
    I do love you both together!
    I love to mark sad faces in fair weather;
And hear a merry laugh amid the thunder;
........

What does that mean, the principle of beauty in all things?
In life, joy is sure being welcomed, but how can we welcome sorrow too? And why?
If there is beauty in all things, does it imply that we can find joy in sorrow, beauty in ugly, poor in rich, good in evil, fair in foul, etc...And in the opposite, those things looked good can have something bad in some ways: sorrow in joy, bad in good, foul in fair, ugly in beauty, poor in rich, etc.....

In his Ode on Melancholy, he tells the truth:

She dwells with Beauty -- Beauty that must die;
  And Joy, whose hand is ever at his lips
Bidding adieu; and aching Pleasure nigh,
  Turning to poison while the bee-mouth sips;
Ay, in the very temple of delight
   Veil'd Melancholy has her sovran shrine,
        Though seen of none save him whose strenuous
   Can burst Joy's grape against his palate fine;
His soul shall taste the sadness of her might,
      And be among her cloudy trophies hung.

Nothing can last forever. Time is always the truth. Joy and pleasure at this moment will be gone next minute. Beauty cannot stay forever and this is the truth. And so does Sorrow and Pain, they are not good but they cannot last forever too and they will be passed. That is true.


In his letter to Benjamin Bailey on March 13, 1818, he shared with him about Religion and also in general that he is very sceptical.
"  I do not think my self more in the right than other people, and that nothing in this world is proveable. I wish I could enter into all your feelings on the subject, merely for one short 10 minutes, and give you a page or two to your liking. ( you can see he is the Chameleon Poet with no self, no identity, he is continually in for others) I have sometimes so very sceptical as to think Poetry itself a mere Jack o' Lantern to amuse whoever may chance to be struck with its brilliance. As tradesmen say everything is worth what it will fetch, so probably every mental pursuit takes its reality and worth from the ardour of the pursuer -- being itself a Nothing...."

But he is sure that the subjective world in every one is the truth for everyone. And his famous quote:
"  I am certain of nothing But the holiness of Heart's Affections and the truth of the Imagination."

So Welcome joy and welcome sorrow!!