Gemara:
The Rabbis taught [in a baraita]: One who enters a bathhouse says as follows: "May it be your will, Hashem, our G-d, to save me from this and similar things, and no destructive thing should occur to me, and if something destructive or related to sin does occur, let my death be atonement for all of my sins." Abayei [from the later period of Amoraim] said: A person should not speak in such a way, for one should not open his mouth to the Satan, as Reish Lakish said in the name of Rabbi Yossi: One should never open up his mouth to the Satan. Rav Yosef said: What pasuk supports this idea? It is written: "We were almost like S’dom; we were similar to Amora" (Yeshaya 1:9). What does the navi respond: "Listen to the word of Hashem, oh officers of S’dom" (ibid. 10).

Ein Ayah: One who is healthy in body and in spirit will not be afraid, and his contemplations will not shake him. Even when he imagines great tragedies, his bravery/confidence will not disappear. That is why in baraita, written at the time when the nation lived in its own Land in relative tranquility and with a spirit that was intact, spoke about potentially tragic things openly without it causing any damage to people’s spirits.
However, in the time of exile (Abayei lived in Bavel), when spirits were weakened, one’s emotions have to be more carefully protected, as people were more apt to lose their confidence, especially when they contemplated frightening scenarios. Once a person with a weakened spirit utters something frightening, it will be difficult for him to remove it from his thoughts. That is why one should not "open his mouth to the Satan," as the characteristics of weakness that have clung to Hashem’s nation due to the pressure exerted by its enemies and its many problems make it necessary to avoid anything that could further frighten it. This is despite the fact that within the personality of the Jewish people there is an innate strength of spirit.
The weakening of the spirit and lack of courage cause the likelihood of deterioration both in the physical and the spiritual realms. When a person’s healthy confidence is shaken, then external ideas of doom are able to infiltrate his mind and impact on his psyche. Therefore, one should even avoid saying anything that could paint a negative spiritual picture of himself and his prospects. When Yeshaya spoke of a comparison of Bnei Yisrael to the people of S’dom, that type of rebuke which displayed a resignation of sorts to the bleakness of the people’s spiritual state actually caused a lowering of the spirit. In that case, it became a self-fulfilling prophecy, prompting the subsequent prophecy to depict them as being deserving of being called the "officers of S’dom."
This teaches us in a broad manner that when giving rebuke, one should not describe a situation, whether from a physical or a spiritual perspective, as fitting of resignation and great darkness. Such words will not elevate the people but lower their dignity to the ground, weaken their resolve, and degrade them. These ideas that lower the spirit have their impact both on an individual and a national level in a manner that the subjects are not able to overcome.