To find the answer to this question, it's necessary to go back to the end of chapter 19 in Harper Lee's novel To Kill a Mockingbird. At the end of that chapter, Dill is crying and upset to the point of being sick because of the way the prosecuting attorney and others are treating Tom Robinson, a black man on trial for raping a white girl, a crime he is innocent of. The context of Dill's emotional outburst is quoted below:
Dill exhaled patiently. “I know all that, Scout. It was the way he said it made me sick, plain sick.”
“He’s supposed to act that way, Dill, he was cross—”
“He didn’t act that way when—”
“Dill, those were his own witnesses.”
“Well, Mr. Finch didn’t act that way to Mayella and old man Ewell when he crossexamined them. The way that man called him 'boy' all the time an' sneered at him, an' looked around at the jury every time he answered—”
“Well, Dill, after all he’s just a Negro.”
“I don’t care one speck. It ain’t right, somehow it ain’t right to do 'em that way."
At the end of chapter 19, Dolphus Raymond says this:
“I know what you mean, boy,” said a voice behind us. We thought it came from the tree-trunk, but it belonged to Mr. Dolphus Raymond. He peered around the trunk at us. “You aren’t thin-hided, it just makes you sick, doesn’t it?"
This exchange continues into chapter 20. Jem and Scout believe Dolphus Raymond is a drunk. He has mixed race children and doesn't care who knows it. He explains that he doesn't usually drink anything but Coca-Cola and only pretends to be drunk because it helps other people latch onto a reason why he lives the way he does. The pessimistic tone he strikes is that when he sees Dill upset to the point of being sick because of the way people are treating Tom Robinson, he says it will only take time for that to change. Consider the following quote:
“Cry about what, Mr. Raymond?” Dill’s maleness was beginning to assert itself.
“Cry about the simple hell people give other people—without even thinking. Cry about the hell white people give colored folks, without even stopping to think that they're people, too."
Dolphus Raymond understands that Dill is crying because Mr. Gilmer is being so condescending to Tom. While Dolphus does all he can to comfort Dill, he tells Dill that he (Dill) had better get used to it, because this is the way that whites treat blacks (and white who chose to associate with blacks). Dolphus doesn't have the optimistic outlook that Atticus and Miss Maudie seem to possess--that life and society can get better if people just learn that blacks are humans with feelings and rights just like whites.