What thoughts does the Grecian Urn arouse in the mind of the poet?Ode on Grecian Urn by John Keats

Our poet/narrator is contemplating a Grecian urn, of
course, and he sees and makes observations about life.  He sees two
scenes"

First, he sees a young man wooing a beautiful young
woman under a tree; they are about to kiss.  The contemplation
is

Bold
Lover, never, never canst thou kiss,
Though winning near the goal yet, do not
grieve;
She cannot fade, though thou hast not thy bliss,
For ever
wilt thou love, and she be
fair!

You may not get to kiss
her, he says, but she will always be beautiful.  There is also a tree in full
bloom whichwill never have to suffer the loss of its leaves, and the young man playing
his pipe will always be young anf have his passion--and his song will never grow
old.

Then he sees a scene of a quaint, empty
town.

What
little town by river or sea shore,
Or mountain-built with peaceful
citadel,
Is emptied of this folk, this pious morn?
And, little town,
thy streets for evermore
Will silent be; and not a soul to tell
Why
thou art desolate, can e'er
return.

What town it is and
why it's empty will forever be a mystery.

The first scene,
while beautiful and everlasting, is sad to him because the promised action of the scene
will never happen:  no kiss, no love, no spring, no
music.

In the second scene, the town will always be
beautiful but it will also forever be empty, not serving the purpose of a town which is
to hold a teeming, living community of people. 

The
narrator is reflecting on the fact that the history (story) of the urn will last far
longer than anything he might write or any story one might live, yet it is not alive. 
Life, while not always so beautiful and perfect, does have love and birth/death and
music and passion--even if it's ugly sometimes.

His final
lines are up for debate and speculation, though they are the most famous and
oft-quoted:

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"Beauty is truth, truth beauty,--that
is all
Ye know on earth, and all ye need to
know."

Keats himself saw the
ambiguity of these lines as a flaw, but that's what poetry is supposed to do--allow for
personal reflection.