To be in a situation in which two of your close friends are romantically involved and to know that one is cheating is a difficult predicament. Ernie might feel it is best not to say anything to Dawn about Bert's cheating for many logical reasons. When Bert revealed to Ernie that he was cheating, Ernie had already promised to keep the promise a secret and not share it with anyone, especially Dawn and would be wrong for Ernie to tell Dawn about the cheating because he would be breaking a promise he had made. Ernie also realizes that Dawn considers Bert to be "the one", and knowing that Bert is cheating could cause Dawn great pain. Ernie, might be influenced by the idea that Dawn might not believe Ernie, feeling that Ernie is just caught up in the rumors. Ernie could have thought that if he revealed the truth, Dawn might confront Bert, and he might deny it by saying "Ernie is just jealous." This might cause the friendship of all three to go downhill, and perhaps this is what Ernie is avoiding. After all, Ernie could also feel that remaining neutral and minding his own business would be the best thing to do, given that they are both are close friends to Ernie.
However, Ernie also has logical reasons to expose the truth. Ernie, himself, does not approve of Bert's cheating. After Bert revealed the truth, Ernie immediately frowned upon the whole idea. Ernie is also aware that Dawn has a false illusion about Bert being the "best" guy out there. If Ernie informs Dawn of the truth, she would be able to reconsider her ideas of Bert and see his true colors. Truth can be considered a very logical reason for Ernie to tell Dawn. In the story, Ernie feels that cheating is wrong and even wants to tell Dawn; however, he is held back by the promise he made to Bert. Ernie might also take into consideration if he was in Bert's situation. He would most likely want Bert, to restrain
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