Answer by taking a quote from the following article:

John Hoyer Updike (March 18, 1932 - January 27, 2009) was an American novelist, poet, short story writer, art critic, and literary critic. One of only three writers to win the Pulitzer Prize for Fiction more than once (the others were Booth Tarkington and William Faulkner), Updike published more than twenty novels, more than a dozen short-story collections, as well as poetry, art and literary criticism and children's books during his career.

The principal themes in Updike's work are religion, sex, and America as well as death. Often he would combine them, frequently in his favored terrain of "the American small town, Protestant middle class", of which he once said, "I like middles. It is in middles that extremes clash, where ambiguity restlessly rules." For example, the decline of religion in America is chronicled in In the Beauty of the Lilies (1996) alongside the history of cinema, and Rabbit Angstrom contemplates the merits of sex with the wife of his friend Reverend Jack Eccles while the latter is giving his sermon in Rabbit, Run (1960). Critics have often noted that Updike imbued language itself with a kind of faith in its efficacy, and that his tendency to construct narratives spanning many years and books--the Rabbit series, the Henry Bech series, Eastwick, the Maples stories--demonstrates a similar faith in the transcendent power of fiction and language. Updike's novels often act as dialectical theological debates between the book itself and the reader, the novel endowed with theological beliefs meant to challenge the reader as the plot runs its course. Rabbit Angstrom himself acts as a Kierkegaardian Knight of Faith. Describing his purpose in writing prose, Updike himself, in the introduction to his Early Stories: 1953-1975 (2004), wrote that his aim was always "to give the mundane its beautiful due." Elsewhere he famously said, "When I write, I aim my mind not towards New York but towards a vague spot east of Kansas." Some have suggested that the "best statement of Updike's aesthetic comes in his early memoir 'The Dogwood Tree'" (1962): "Blankness is not emptiness; we may skate upon an intense radiance we do not see because we see nothing else. And in fact there is a color, a quiet but tireless goodness that things at rest, like a brick wall or a small stone, seem to affirm."

Why did he like middles
It is in middles that extremes clash, where ambiguity restlessly rules."