Rabbi Mayer Twersky
Shabbos Shabboson he lochem v'inisem es nafshoseichem chukas olam - It is a Sabbath of complete rest for you, and you shall afflict yourselves; an eternal decree (Vayikra 16:31, Artscroll translation)
Inui nefesh, affliction, according to the Torah she'b'al peh, refers to fasting. Abarbanel suggests a remarkable additional interpretation. Yom Hakipurim is, of course, a day devoted to teshuva. There is nothing casual about teshuva. Cheit represents betrayal, failure, hypocrisy, hubris, foolishness and rebelliousness. Cheit signifies the opportunities of life squandered. Teshuva involves recognition of cheit and its magnitude. And thus the self-awareness induced by teshuva is excruciating. Ultimately, teshuva culminates in the joy and elation of kapara and rapprochement with Hakadosh Baruch Hu. But the path is paved with anguish.
When the Torah commands inui nefesh on Yom Hakipurim it speaks of this penitential, existential anguish as well.
Abarbanel's beautiful interpretation with its sensitive depiction of teshuva provides us with a means to monitor our teshuva efforts. If we are coasting along, singing ashamnu and "klapping" al cheit but not feeling anguish, we have yet to hit the mark. "Aval hacharata u'sheviras ha'lev v'ha'bechi zeh nachutz me'od v'ha'marbeh b'bechi b'mistarim meshubach - but [feeling] remorse, [having] a broken heart and crying are necessary. And one whom in private, cries effusively is praiseworth" (Nodah B'Yehuda, Mahadura Kamma, Orach Chaim 35.)