Why did Nick need proof to believe Gatsby's story? In the great Gatsby chapter 4 Anyone who answers right will get brain listed :)

Little Montenegro! He lifted up the words and nodded
at them—with his smile. The smile comprehended Monte-
negro’s troubled history and sympathized with the brave
struggles of the Montenegrin people. It appreciated fully
the chain of national circumstances which had elicited this
tribute from Montenegro’s warm little heart. My increduli-
ty was submerged in fascination now; it was like skimming
hastily through a dozen magazines.
He reached in his pocket and a piece of metal, slung on a
ribbon, fell into my palm.
‘That’s the one from Montenegro.’
To my astonishment, the thing had an authentic look.
Orderi di Danilo, ran the circular legend, Montenegro,
Nicolas Rex.
‘Turn it.’
Major Jay Gatsby, I read, For Valour Extraordinary.
‘Here’s another thing I always carry. A souvenir of Ox-
ford days. It was taken in Trinity Quad—the man on my left
is now the Earl of Dorcaster.’
It was a photograph of half a dozen young men in blazers
loafing in an archway through which were visible a host of
spires. There was Gatsby, looking a little, not much, young-
er—with a cricket bat in his hand.
Then it was all true. I saw the skins of tigers flaming in
his palace on the Grand Canal; I saw him opening a chest of
rubies to ease, with their crimson-lighted depths, the gnaw-
ings of his broken heart.
‘I’m going to make a big request of you today,’ he said,
pocketing his souvenirs with satisfaction, ‘so I thought you