Likutei Moharan 50:2/ליקוטי מוהר"ן נ׳:ב׳

12. Know, too, that the arrogant people of the generation — they are the dogs, as in, “and the dogs are brazen.” They rise and take issue with the prayers of a Jew who has yet to completely rectify his brit .

2This is as our Sages, of blessed memory, taught (Taanit 11b): When a rabbinical student sits in fasting, [he deserves to have] a dog consume his meal. For certainly a righteous person’s eating is very precious since it brings contentment to the holy soul, as it is written (Proverbs 13:25), “A tzaddik eats to satisfy his soul.”

3But because this rabbinical student starves his soul, and knows not how to satisfy his <holy> soul, [he deserves to have] a dog consume his meal. For also of them it is written (Isaiah, loc. cit.), “they never know satisfaction.” He, too, does not know how to bring contentment to his precious soul. This is why a dog is called a companion (Rosh HaShanah 4a), because it appears as a result of a desire for cohabitation, as explained.

4And it is particularly when a person fasts though not required to, that [he deserves to have] a dog consume his meal. On the other hand, someone who is required to fast, should certainly fast. It is a <great> mitzvah to do so.

5This is the aspect of “sits in fasting.” Specifically “sits,” because sitting points to an absence of change, as explained <elsewhere>.