Modernism can be described as an artistic response to the industrial revolution. After industrialization had changed the landscape of the Western world, modernism emerged. The industrial revolution changed more than modes of production. It changed modes of living and led, directly or indirectly, to a new global phase of bureaucratic nationalism and two World Wars.
The significant shifts taking place around 1900 also included the advent of the automobile; the rapid growth, development and popularity of cinema; and the intellectual influence of Sigmund Freud. Technology was suddenly part of every person's life (to exaggerate a bit, but only a bit) and reality itself had been brought into question by Freud's theories. The old world of slow travel, agrarian life-styles and no electricity was being destroyed.
What is identified as the characteristic themes or concerns of the modernist period (a general pessimism about the state of the world, a rejection of society’s certainties, a sense that only the rebel artist is telling the truth about the world) were simply “in the air” of the times...(eNotes)
An idea emerged within modernism that every individual lived in his or her own subjective, psychologically filtered reality.
A major element of modernism in literature is the use a of stream-of-consciousness narrative form, which emphasizes the ways in which individual's perceive reality in distinct and idiosyncratic ways. Another manifestation of a fracturing or fragmentation of identity and experience in the modernist age comes in the use of distinctly different forms and types of writing within single works.
T.S. Eliot used this method in his poetry, mixing languages, dialogues, couplets and other forms in his poetry. John Dos Passos became famous for doing something similar in his novels - using advertising language, first-person narratives, and third-person narratives within single novels to create a formal palimpsest of layered prose forms. For both the poet and the novelist, the method was useful in rendering an experience of "high-volume" and variously sourced influences on the individual.
Themes of alienation, a personal (often troubled) relationship with history, and of social decay are prevalent in modernist works of literature (eNotes).