The Talmud says:
תנו רבנן: "מִי יִתֵּן וְהָיָה לְבָבָם זֶה לָהֶם." אמר להן משה לישראל: "כפויי טובה בני כפויי טובה!" בשעה שאמר הקב"ה לישראל מי יתן והיה לבבם זה להם, היה להם לומר תן.
The rabbis taught: “Who can assure that their heart will remain theirs …” – Moses said to the Children of Israel: “Ingrates, sons of ingrates!” When God said to Israel “Who can assure…” they ought to have said “assure it for us!” 
I was confused when I read this. The Gemara calls the Jewish People כפויי טובה, ingrates, on account of their neglecting to request that God “assure them their hearts.” Why in the world would someone be called an ingrate for not asking for something?!
Then I saw that Tosafot is bothered by this too, and says the following answer:
לכך קראם כפויי טובה שלא רצו לומר אתה תן לפי שלא היו רוצים להחזיק לו טובה בכך.
For this reason he called them ingrates: Because they did not wish to say “give” for they did not wish to be grateful to him.
This is a fascinating insight. Oftentimes people reject gifts or even assistance because they say that they do not wish to be takers. But how many are simply motivated by a sense of unease with the concept of gratitude? How many do not take, simply because taking betrays the secret that they are human; that they actually need others?
We all know the proverb, שׂוֹנֵא מַתָּנֹת יִחְיֶה, he who hates gifts will live. There is an obscure statement made by the Talmud about this verse, which goes as follows:
ואמר רב חסדא: איזהו "שונא מתנות יחיה"? זה הרואה טרפה לעצמו.
Said R. Ḥisda: Who is “one who hates gifts”? This is one who renders his own animal unkosher. 
What does this Gemara mean?
In light of the above Tosafot, perhaps one could explain as follows. The Gemara is asking: Who is the honest “one who hates gifts,” and who is the one who rejects them out of a character flaw? Who is truly not a taker, and who is simply an ingrate?
One who is unwilling to acknowledge another’s contribution to his life is one with his head in the clouds. Such a person is unwilling to acknowledge that there is anything in the world that matters other than himself. When such a person has a question whether or not his animal is kosher, will he ever be extra stringent? Surely not! The world is there for him and for his sake only. Halakha is there to serve him as well! He is the classic נבל ברשות התורה, the degenerate with the “permission” of the Torah. Such a person would certainly be lenient when ruling regarding his own possessions.
Therefore, says the Gemara, if you see someone who is not a taker, and you wish to know the nature of this habit and how honest the person is about it – look at what he does when he has a halakhic doubt with regard to his own things. If he is not quick to be lenient, then you know he is the real deal.
It is no wonder that when the convert came before Hillel and asked him to teach him the entire Torah while standing on one foot, Hillel’s reply was “What you hate, do not do unto your friend; the rest is explanation.” The way we approach interpersonal relationships is deeply intertwined with the way we view ourselves, which in turn is the root cause of the way we act in our relationship with God as well.
Kudos to NB for giving me part of the idea for this post.
 Deuteronomy 5:26
 Avodah Zarah 5a.
 See ibid for the Talmud’s explanation of what exactly this refers to.
 Ibid s.v. כפויי
 Proverbs 15:27
 Ḥullin 44b.
 See Rashi for an alternative explanation - הרואה טרפה לעצמו ודאי שונא מתנות אחרים הוא שאף על שלו אינו חומד להכריע להיתר.
 Shabbat 31a