The last part of the novel hints at Scout's optimism.

At the end of Chapter 29, Scout finally meets Boo Radley face to face. The text tells us that tears came into her eyes when she spotted Mr. Radley leaning against the wall in the corner. Why? It was Boo Radley who saved Scout and Jem from certain death at Mr. Ewell's hands.

In prior chapters, we learn that Boo Radley had always graced Maycomb's collective imagination as a "malevolent phantom" of sorts. Any crime committed in Maycomb was summarily attributed to Boo Radley. When chickens and household pets were mutilated in a series of "morbid nocturnal events," everyone ascribed the crime to Boo Radley.

As for the children, Scout, Jem, and Dill played games featuring Boo as a malicious character. So, all things considered, Scout never envisioned that she would enjoy friendly interactions with this larger-than-life boogeyman.

However, at the end of the story, Scout not only meets Boo, she even converses with him. Later, Scout escorts Boo home. During her short time with Boo, Scout gets the opportunity to witness Boo's humanity and his vulnerability. Although she never sees him again, Scout's interactions with Boo imbues her with hope and optimism for the future.

The story ends on a hopeful note, and we get the idea that Scout's optimism arises from her knowledge that goodness and integrity have not disappeared from society.

Scout is one of the chief characters in Harper Lee's "To Kill A Mockingbird".  She possesses an unusual sense of optimism, which is a tendency to see the bright side of things, instead of pessimism, which is to see the bad side of things.  As the novel develops, Scout encounters plenty of evil, which is countered with good.  By novel's end, she has, under the tutelage of Atticus, her father, learned a great truth about the world she lives in.  While there is a great potential for evil to take place, there is also a great potential for good things to take place as well.  This is an important development in the book, as Scout ages two years in the development of the story.  At age five, going to age seven, the potential to develop either philosophy is there, depending upon the influence of one's important adult figures, such as parents, community, and school.