For the Arkenstone ...is worth more than a river of gold in itself...

The Arkenstone is the "fairest of all" the dragon's hoard, and Biblo thinks "there could not be two such gems, even in so marvelous a hoard, even in all the world."  So it is of real value to Thorin, priceless, really.

For the Arkenstone of my father...to me it is beyond price.

As Thrain's stone, it is of sentimental value to Thorin. We are also told that the dwarves dug it from the heart of the mountain, beneath the roots of the Mountain, and that it was cut and fashioned by the dwarves. So, for Thorin, this stone represents not only his father, not only his ancestral and childhood home, but also the peak of the craftsmanship of the dwarves.  They will never fashion something more beautiful.

I think the removal of the Arkenstone from Lonely Mountain also represents to Thorin his inability to restore all things -- the heart of the mountain is gone, the best of the dwarves is gone, his father's legacy is diminished.

Add to all this the possible enchantment of both the dragon's hoard and this stone in particular.  Remember, "Bilbo's arm went towards it drawn by its enchantment."  It also says, "the lust of it was heavy on Thorin."

In light of all this, why would a friend of Thorin's withhold this treasure from him?  The idea seems a little paranoid, but just because you're paranoid, doesn't mean they're not out to get your treasure!

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Roads go ever ever on
  Under cloud and under star,
Yet feet that wandering have gone
  Turn at last to home afar.
Eyes that fire and sword have seen
  And horror in the halls of stone
Look at last on meadows green
  And trees and hills they long have known.

The wanderer sleepeth.  Now soft be his pillow!