Bitterness perhaps against their mortality. The 'mortality' of men had been existent since before their creation. They just needed an excuse, remember that Tolkien says 'All men are fallen', both in his Letters and in the Athrabeth.
What was the reason behind this claim of Men? Surely, there is some reason (although in the Athrabeth Men do loathe to tell that to elves) what are your thoughts about this claim, and what do you think is the reason behind this claim
Yes. Looking at the age spans of some of the Beorian lords, they often numbered 90+ years, and Andreth comments on it. I think that in a way they had found 'paradise' in pre-Bragollach Beleriand and they could interact with the crème de la crème of the Quendi, the Eldar (As opposed to the Avari) and this had an effect on them. But Sador, Hurin's servant comments on this, the 'flame' (fea?) of men burning quicker when they interact with the Eldar:
Men's life span was shorter before they reached Beleriand? Do you believe this claim to be true? If yes, then why?
Unfinished Tales; Of Turin Turumbar
In their light we are dimmed, or we burn with to quick a flame...
This of course, could be a poetic reference to their low life expectancy in comparison to Elves.
The Valar, apart from Ulmo maybe, had little interaction with men, Finrod seems to think men are 'too great' for them to handle. Read the part about Andreth's fall carefully, and you will see how Eru allows them to make their own discoveries and decisions.
Why? Did the Atani learned most of their craft from the Dark Elves and the Nandor? or were they born with the knowledge of Middle-Earth? And if not then the development of a whole civilaztion is a long prcoess while the men achieved it in about three hundred years? Had the Vala or Eru already (if Vala had the power) enriched the brains of Men with the knowledge of Middle-Earth?
On Elvish help before they entered into Beleriand. It's is probable that any Nelyarin Avari would’ve taught them lore (As Tolkien comments on) but the Tatyarin Avari, who were not as friendly as their Moriquendi counterparts would've been unfriendly to them and judging by the reaction of the Nandor of Ossiriand, in which they said they would afflict men unless they left, they would've received little help from any Eriadorin Nandor, or any that they encountered on their journey. 'Of Dwarves and Men' comments on the kinship between the tongue of the Easterlings and the Easterlings and about various alliances between the Longbeard Dwarves and pre-Marachian tribes in Rhovanion, so they could've learnt lore from a combination of Nelyarin Avari and Dwarves.
I don't understand why are you using outdated names? 'Ingold' as a name for Finrod only existed in the pre-LoTR Quenta's of HoME 4 and 5.
What might the answer be? Since the Answer of Ingold seems so logical
As Eriol said, Men were supposed to have awoken much further back, in Tolkien’s latter writings. I believe this is the case in the Athrabeth as in the story of the downfall Melkor puts on a fair semblance and the Silmarillion tells us that after his fleeing from Formenos and destruction of the Trees he could never put on a fair semblance again. Also the time-line for it cannot possibly allow for the men to 'fall' and then for some to rebel and migrate, and reach Beleriand. That couldn’t happen in 300 odd years.
Men awoke in 1 A.O.S. (age of Sun)
Curiosity is insubordination in its purest form-Vladimir Nabokov
Do not read as children do to enjoy themselves, or, as the ambitious do to educate themselves. No, read to live. -Gustave Flaubert
We are not provided with wisdom, we must discover it for ourselves, after a journey through the wilderness which no one else can take for us, an effort which no one can spare us.-Marcel Proust