Modernist attitudes are reactions against the previous two periods of Victorianism and Romanticism.
Romanticism is centered on the individual. Specifically, it is focused on the emotions of the individual. That is a polar shift from the Age of Reason which preceded it. Age of Reason emphasizes logic over emotion and the good of the group over the good of the individual. Romanticism emphasizes an individual's emotions. Additionally, Romanticism places great importance on nature and its power to give transcendent knowledge to people who are willing to "be one with nature." It's an incredibly positive literary period. It's full of awe and wonder at all that Earth has to offer. It's a very active form of literature, too, because it emphasizes carpe diem. Lastly, there is a great deal of imagination expressed by Romantic authors.
Realism (I'll get to Victorianism) is the following period. It is a reaction against the Romantics. Instead of focusing on the individual, emotion, and imagination, the realists often focus heavily on objectivity. A realist wants to describe something exactly how it is. No embellishment. A realist will also focus on the normal, everyday activities. Similar to Romanticism though, realists still often focus on an individual. For example, Twain's novels often focus closely on a few key characters. As does "Red Badge of Courage" by Stephen Crane.
Victorianism is a kind of offshoot of Realism. It's a nicer read in my opinion, because it isn't quite so "dark." A realist often has a lot of characters die off, because "that's just how it is." Victorianism, on the other hand, tends to be more uplifting, because of the specific individuals being focused on. Like realism, it will focus on regular, everyday life, but it will be focused on a character's attempts to achieve upward mobility through that life. For that reason, it tends to read more positively than pure realism. The authors often idealize life and make appeals to idealistic love, truth, justice, etc. For example Dickens's "Great Expectations."
Modernists are my least favorite. I feel that they take the objectivity of realism and the hope of Victorianism's upward mobility and mix the two together. That doesn't sound so bad, but the product is an entire genre of literature where characters are hoping to achieve something in life, only to have it constantly thrown back in their faces. It's a depressing genre. "Of Mice and Men," "Barn Burning," "The Great Gatsby," "A Rose for Emily," etc. are just a few of the stories and novels that focus on the lost hope of so many characters.
Modernists reacted against Romanticism's hope and positivity. Modernists reacted against Victorianism's realistic hope of upward mobility. Instead modernists focus on the decline of society and characters. The writing does tend to focus on a single character or two, but it is focused on their internal struggles against a societal machine that exists to oppress and destroy.