Halakhah zu Ejchah 3:39

When you fast, you should not indulge in enjoyment, nor act frivolously. You should not go around happy and cheerful, but rather with anxiety and sorrow,⁷You should be careful to control your anger and to conduct your business activities in such manner, as not to become angry. (Ibid. 568:50) as it is said: "Why should a living man bemoan? A man, because of his sins!" (Lamentations 3:39).

One who is fasting, whether for his troubles or his dreams or whether he is fasting with the community for their troubles, he should not experience pleasure, nor should he act lightheartedly, nor should he be happy or in good moods. Rather, he should worry and mourn, following what is written "Of what should a living man complain?" ([Lamentations 3:39]).