The three reasons Atticus should not defend Tom Robinson are as follows: it brings antisocial and professional stress to his life; it brings antisocial and adversity to his children's lives; and he won't win the case anyway. First, the unneeded stress to Atticus himself is compounded because he is older (in his fifties), a widower, and is raising two children. When he first discusses the situation with his daughter Scout, he says the following:
". . . simply by the nature of the work, every lawyer gets at least one case in his lifetime that affects him personally. This one's mine, I guess" (76).
Atticus is trying to tell Scout about how this is affecting his work and his life so that she will behave like an upstanding young lady, thereby, not causing him more things to worry about. He also takes the time to prepare her for the adversity she will probably continue to face because of the trial.
"You might hear some ugly talk about it at school, but do one thing for me if you will: you just hold your head high and keep those fists down. No matter what anybody says to you, don't let 'em get your goat. Try fighting with your head for a change. . . it's a good one" (76).
Thus, some would argue that if Atticus simply refused to take the case, then he and his children would not have had to deal with such stress in their lives. In fact, Scout gets into fights with Cecil Jacobs and her cousin Francis to defend her father's honor; and, Jem loses his head and takes out Mrs. Dubose's flowers because she calls Atticus horrible names.
Finally, because the case is lost before it begins, Link Deas, a businessman in town, tells Atticus that he's got everything to lose from defending Tom Robinson. Atticus doesn't think so and responds as follows:
"Link, that boy might go to the chair, but he's not going till the truth's told. . . And you know what the truth is" (146).
The above quote demonstrates more social and professional stress applied to Atticus, but it also shows that he knows that the trial is a lost cause even before it begins. A black man's word against a white man's in Alabama in 1935 is sure to be lost because of the unequal balance of power in the judicial system at the time. Atticus knows this, but he does his best to give Tom Robinson the best defense possible anyway.