What is a life lesson that one learns from To Kill a Mockingbird? What quotes from the book support that lesson?

To Kill a Mockingbird is one of those novels that famously deals with life lessons. In fact, it gets a lot of criticism for being overly direct (not subtle) and sometimes even preachy about life lessons! So you have lots of them to choose from. To find one you like, try skimming the book until you see Atticus doing a lot of talking. He's basically the embodiment of morals throughout the story.

Here's a particular life lesson that I enjoy finding in the book. Although it's easy to say that everyone deserves kindness, it's harder for people to actually do it: it's hard for us to follow that golden rule with consistency. So we can't just tell ourselves that we're good people and leave it at that. We have to pay attention to how we're treating people and work actively to extend humane kindness to everyone, especially to people we don’t particularly like.

Some quotes that illustrate this life lesson are as follows.

1. Here's something Scout says to Jem as she's talking about her teacher:

"'Well, coming out of the courthouse that night Miss Gates was—she was goin' down the steps in front of us, you musta not seen her—she was talking with Miss Stephanie Crawford. I heard her say it's time somebody taught 'em a lesson, they were gettin' way above themselves, an' the next thing they think they can do is marry us. Jem, how can you hate Hitler so bad an' then turn around and be ugly about folks right at home—'"

Scout means, how can you condemn Hitler's treatment of the Jews and then fail to treat black people with kindness? It's inconsistent, yet Miss Gates probably thinks of herself as a kind person anyway!

2. Although Aunt Alexandra is noted for her excellent manners, we also know this about her:

"Let a sixteen-year-old girl giggle in the choir and Aunty would say, 'It just goes to show you, all the Penfield women are flighty.' Everybody in Maycomb, it seemed, had a Streak: a Drinking Streak, a Gambling Streak, a Mean Streak, a Funny Streak."

If you asked Aunt Alexandra, she would definitely tell you that she is a nice person with excellent manners towards others, and yet she constantly makes unkind comments about people behind their backs. 

3. After the jury deems Tom guilty of the crime he couldn't have possibly committed and Jem wants to know how the jury members could have possibly done such an unfair thing, Atticus says:

"'I don't know, but they did it. They've done it before and they did it tonight and they'll do it again and when they do it—seems that only children weep.'"

What Atticus means is that it's a nasty part of human nature to treat people you don't like in an unkind, unfair, inhumane way, and that we're all so used to this that it's only kids (the most innocent people) who see how wrong it is.