The Gemara gives the source of the Yidden being ingrates as they complained about the mon. This however seems strange as the baraisa seems to say clearly that they were ingrates as they refused the opportunity for Hash-m to remove their yetzer hora.
furthermore, if you are going to say that it is just bringing another source, then A) that seems unnecessary and B) It's not bringing all the sources any way (see ben yehoyada e.t.c. who explain the braisa saying benei kefuyei toivah (plural) as also refering to the dor haflagah and the pessukim there which the gemara could have brought as additional sources to the Yidden being 'benei kefuyei toivah' and these pessukim would seem to be even more necessary to explaining the braisa as they explain who the benei kefuyei toivah are...)
someone, Melbourne Australia
I want to try and answer this question, bs'd, with the help of one of the parables of the Dubner Magid zt'l. I saw this in a Sefer of the Mishlei Magid mi'Dubna on the Parshas ha'Shavu'a (in Parshas Va'eschanan, Devarim 5:26).
1) The Dubna Magid asks a Kashya on the Gemara which says that Yisrael should have said to Hash-m, "You give!" They should have asked Hash-m that he would give them the Fear of Heaven. But how could have they asked for this? Doesn't the Gemara say that "everything is in the hands of Heaven with the exception of Yir'as Shamayim" (Berachos 33b)? A person has free choice and there is no guarantee that he will always fear Hash-m!
2) The Dubna Magid answers his question, of course, with a story. A brilliant young Torah student married the daughter of the wealthiest member of the community. The father-in-law was so proud of the Chasan that in the first days after the wedding he supplied his every need with great joy. The new husband was of course very grateful for the wonderful warm reception that his bride's family extended to him, but he was also wise enough to remember that it is possible that the enthusiasm of the new relationship might cool down after the novelty had worn off. So he asked his father-in-law to give him now all the money that he had committed himself to give him in the coming years.
3) The parable is that everything depends on how one starts off. If, at the beginning of one's life, one prays genuinely that Hash-m will always help us to fear Him, this prayer has much more power because we are telling Hash-m right at the beginning that we always want His help. This is comparable to the young son-in-law who sees that he is loved now and wishes to ensure that this love will always remain. What one prays for at the beginning shows how one wants to continue.
4) Now we can understand why the Gemara mentions the two instances of Kefuyei Tovah, both refusing to ask Hash-m to remove their Yetzer ha'Ra, and also complaining about the Man. At the time of receiving the Torah, Bnei Yisrael were on a very high Madreigah. They were comparable to the brilliant Torah student who has just married and has a great future ahead of him. He is inclined to become very proud and if he does not ask his father-in-law for security that he will always help him, this is because he is ungrateful to him and does not appreciate all his efforts. If the son-in-law lets things go by and a few years later his behavior deteriorates and he falls out with his in-laws, what happens later on will be the noticeable incidents of ingratitude. It is true that he could have solved the problem right at the beginning, but it is only later on that the Kefuyei Tovah becomes apparent to everyone.
5) This is why the Gemara mentions both complaining about the Man and refusing to ask Hash-m for help against the Yetzer ha'Ra. They could have avoided everything if they would have asked for help right at the beginning, at Matan Torah. But this was only a passive, subtle failure. Therefore, the Gemara mentions the famous failure, by which time everyone could see the results of their original Kefuyei Tovah.
Kol Tov,
Dovid Bloom
While this answer fits very well with the Chiddushei Aggados who explains that kefuyei toivah means they do not even appreciate that what they got is good, it does not fit as well with Rashi and Tosafos who understand it as even though the person appreciates it but nonetheless refuses to recognise it so as to not owe a favor (that is my understanding of rashi and tosafos). According to Rashi and Tosafos, not saying Atah Tein is clearly considered kefuyei toivah (this is very clear in Tosafos).
I think it can fit with Rashi and Tosfos as well. The brilliant son-in-law knew that his new family was being very generous to him but he did not want to acknowledge this. He thought that he deserved it, and may well have reasoned that the father-in-law is being so nice to him only because he is so proud that his daughter married such a wonderful person. Because the Chasan thought that he deserved all the honor and bounty coming his way, he was too haughty to say "Tein Atah" because he expected the help to be indefinite. If he would have appreciated what his in-laws were doing for him, he would have understood that it should not be taken for granted.
Kol Tuv,
Dovid Bloom