Our Gemara on Amud Beis discusses the psychology of why there is an obligation to make an oath when there is partial admission, even though if there is total denial, there is no obligation. At first glance, this does not make sense. Why did somebody who is decent enough to make a partial admission be less credible than a person who totally denies? But of course, human behavior does not follow logic; it also follows emotions. Therefore, the reasoning is as follows:
אָמַר רַבָּה, מִפְּנֵי מָה אָמְרָה תּוֹרָה: מוֹדֶה מִקְצָת הַטַּעֲנָה – יִשָּׁבַע? חֲזָקָה – אֵין אָדָם מֵעִיז פָּנָיו בִּפְנֵי בַּעַל חוֹבוֹ;
Rabba says: For what reason did the Torah say that one who admits to part of the claim brought against him takes an oath with regard to the rest of the claim, which he denies, whereas one who denies the entire claim is not required to take an oath? Rabba answers: The oath of partial admission is based on a presumption with regard to the defendant’s behavior. There is a presumption that a person would not be so brazen as to stand before his creditor and deny his debt when his creditor knows that he is lying.
וְהַאי, בְּכוּלֵּיהּ בָּעֵי לְמִכְפְּרֵיהּ לֵיהּ, וְהַאי דְּלָא כַּפְרֵיהּ – מִשּׁוּם דְּאֵין אָדָם מֵעִיז פָּנָיו בִּפְנֵי בַּעַל חוֹבוֹ; וּבְכוּלֵּיהּ בָּעֵי דְּלוֹדֵי לֵיהּ, וְהַאי דְּלָא אוֹדִי לֵיהּ – אִישְׁתְּמוֹטֵי הוּא דְּקָא מִשְׁתְּמִיט לֵיהּ – סָבַר: עַד דְּהָווּ לִי זוּזֵי וּפָרַעְנָא לֵיהּ; וְאָמַר רַחֲמָנָא: רְמִי שְׁבוּעָה עִילָּוֵיהּ, כִּי הֵיכִי דְּלוֹדֵי לֵיהּ בְּכוּלֵּיהּ.
Rabba continues: And this one who admits to part of the claim would want to deny all of it, and the only reason he does not deny all of it is because a person would not be so brazen before his creditor. And in fact, he would want to admit to all of the claim to him. And the reason that he did not admit the whole claim to him and say that in fact he owes him the entire sum is that he was evading his obligation temporarily. The debtor is short of money and he thinks: I will pay my creditor as much as I can afford now, and I will evade paying the rest until I have enough money, and then I will repay him the rest, to which I have not yet admitted. Therefore, the Merciful One states: Impose an oath on the debtor in order to induce him to admit all of the debt to the creditor.
In other words, really the fellow is fully obligated, but he does not have the money, and out of guilt he makes a partial admission. This alone is suspect, and therefore to flush him out, we force him to make an oath. Since he is not a brazen thief or liar, once he has to make an oath, he will be induced to come clean.
There is a Chassidic metaphor derived from this legal ruling (see Likkutei Halakhos Choshen Mishpat, Laws of Loans 3:4.) The Torah takes an understanding and merciful approach toward a person tempted by circumstances toward dishonesty. Instead of calling him a liar, we actually believe that he wants to tell the truth. We surmise that his partial admission is his own the way of trying to rationalize his conflict between his shame of being unable to pay his debt while also his shame in denying the debt. So instead of completely mistrusting him, we will get an oath because deep down we do trust him and we know that if he is lying, he will now come forward.
Rav Nachman says this is also true in regard to our sins. Sometimes we are not able to fully admit to God (and maybe ourselves!) Our sin. We will rationalize and make partial admissions. Even so, God accepts this partial admissions as part of our process toward recognizing the full truth that we need to face.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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