Empathy and a sense of underlying equality are two traits that drive Atticus. Miss Maudie says at one point that Atticus gave up shooting, despite his great skill, because he discovered he had an unfair advantage over other creatures.
This sense of equality pervades his character and his actions, as he encourages his children also to recognize the fact that everyone has feelings (which can be hurt) and everyone is capable of being a good person, of growth, learning and honor.
Atticus Finch is probably my favorite character in literature, but it occurs to me that while I've always admired his strong moral character, integrity, and conscience, I've never really thought about what motivated him, which led me to wonder about his parents. We know that his sister, Alexandra stayed at the family's ancestral home, Finch's Landing, and that Atticus paid for his brother Jack's education; however, we don't know anything about the trio's parents, and what they might have done or not done to raise these three. Someone or something instilled the profound sense of right and wrong in Atticus Finch. Even Alexandra, who is probably the least sympathetic of the three, but even she proves to have some character and inner strength at the end of the novel. It would be interesting if once could ask Harper Lee about the lack of background information on Scout and Jem's grandparents and what might have shaped her decision to limit the discussion of previous Finch generations to Simon Finch.
Atticus's general motivation for his actions and beliefs in life are laid out in implied form in the first few pages of the story. Using excellent writing craft, Lee gives us implied reasons for what Atticus is, why he is that way, and what we can expect from him. His first motivation is his moral background, which he gained from his early ancestor, Simon Finch, who was a staunch Methodist (and who only forgot Methodist teaching on one or two points ...). His second motivation is his respect for the law, which is strongly implied since he is a lawyer. His third motivation is the absence of greediness, as he chose to open his law practice in his home county and as he "practiced economy more than anything" in the early years of law practice. His fourth motivation is his generosity and sense of brotherhood. This is implied when his younger brother wanted to go up to Boston to study medicine; Atticus paid for his education. His fifth motivation is the the family connection he felt with "nearly every family in the town." These are the most critical of the things that motivate Atticus in all his actions, thought, decisions, and relationships with the townspeople.
Atticus Finch represents what all lawyers should be--ethical. Unlike many defenders, he takes cases because of his sense of justice and ethics, standards by which he conducts all his actions. As Miss Maudie tells Jem and Scout, "Atticus Finch is the same inside his house as he is on the public streets."
Atticus takes on the case of Tom Robinson so that his children will not suffer from "Maycomb's disease" of class and racial bias. Always he sets an example of ethical and Christian behavior. For instance, having been raised as a gentleman in the true sense of the word, Atticus tilts his hat to Mrs. Dubose and remains sanguine towards her no matter the venom of her speech. Likewise, he insists that his children be polite, even punishing Jem for retaliating against her for her cruel words about his father. When she dies, Atticus calls her "the bravest woman I know" because she withdrew herself from morphine in her final days, enduring great pain in order to face death on her own terms. In another example, when Bob Ewell spits in his face, he "turns the other cheek" and he does not condemn the man, instead explaining to his children why Ewell felt the need to act as he has done.
Above all, as an ethical man, Atticus Finch believes in the truth. These words exemplify this belief,
"When a child asks you something, answer him, for goodness sake. But don't make a production of it. Children are children, but they can spot an evasion faster than adults, and evasion simply muddles 'em."
Atticus is motivated by a moral code that he lives his life by. I agree with bullgatortail that Atticus is worried about how his children will perceive him. However, he also has a strong sense of justice and little time for bigotry. When Tom Robinson is accused of raping Mayella Ewell, Atticus agrees to defend Tom even though he does not believe he has much chance of winning. Atticus sees Tom not as a man of color, but as a man with the right to a fair trial. The reader learns that Judge Taylor chose Atticus on purpose, knowing that if nothing else, Atticus' legal representation would have Tom's best interests at heart. Atticus is also a man of faith. He tells Scout:
This case, Tom Robinson's case, is something that goes to the essence of a man's conscience—Scout, I couldn't to to church and worship God if I didn't try to help the man.
Atticus, as Jem puts it in Chapter 10, is a gentleman.
Naw, Scout, it's something you wouldn't understand. Atticus is real old, but I wouldn't care if he couldn't do anything—I wouldnt' care if he couldn't do a blessed thing...Atticus is a real gentleman, just like me.
Jem may be biased in believing this, but Atticus proves it in the way that he treats Mrs. Dubose. In Chapter 11, the reader discovers that she is a nasty, critical woman, but Atticus knows that her life has not be easy, especially of late because of her illness. He always demands that the children treat her with respect.
"Easy does it, [Jem]," Atticus would say. "She's an old lady and she's ill. You just hold your head high and be a gentleman. Whatever she says to you, it's your job not to let her make you mad."
He even sets an example for Jem to follow:
When the three of us came to her house, Atticus would sweep off his hat, wave gallantly to her and say, "Good evening, Mrs. Dubose! You look like a picture this evening."
Atticus is motivated to be the man he is by his sense of gentlemanly behavior and his convictions as to the rights of all men, regardless of their color.
Atticus is one of the all-time great literary characters, in my opinion. I think his motivation is to rise above the ordinary and not let irrational thinking get in his way and direct his thinking. Unlike many of the people of Maycomb, Atticus does not form biased opinions about other people. He has an uncanny patience and ability to see the motivations of others. This quote exemplifies his philosophy of looking at and judging others.
You never really know a man until you stand in his shoes and walk around in them.
Atticus is motivated to take on the Tom Robinson case because of his children: "But do you think I could face my children otherwise?" He demands that Scout return to school because he wants her to have a better education than he had (Atticus was educated at home). And, at the end of the story, he is willing to let Jem take the blame for Bob's death because "I don't want him growing up with a whisper about him...," that 'his daddy paid a mint to get him out of that.' "
Atticus is motivated by a sense of justice. He feels strongly about equality and that is why, even though he probably knows that he will lose the case, he takes it on and defends Tom Robinson to his best ability.