Rabbi Yose says: A person is not permitted to afflict himself with fasts, lest he become dependent on his fellow creatures and the creatures show him no mercy. What is Rabbi Yose's reasoning? As it is written, "And the human became a living being" (Genesis 2:7). The Torah said: the soul that I have given you - keep it alive.
Rabbi Yose Warns Against Afflicting the Soul With Fasts
Yalkut Shimoni on Torah 20:14
רַבִּי יוֹסֵי אוֹמֵר, אֵין אָדָם רַשַּׁאי לְסַגֵּף עַצְמוֹ בְּתַעֲנִיּוֹת שֶׁמָּא יִצְטָרֵךְ לַבְּרִיּוֹת וְאֵין הַבְּרִיּוֹת מְרַחֲמִין עָלָיו. מַאי טַעְמָא דְּרַבִּי יוֹסֵי, דִּכְתִיב "וַיְהִי הָאָדָם לְנֶפֶשׁ חַיָּה", אָמְרָה תּוֹרָה, נְשָׁמָה שֶׁנָּתַתִּי לְךָ הַחֲיֵה אוֹתָהּ.