Finch and Violet are dealing with complex problems, including grief and mental illness. Fortunately for us, they're really good at explaining those uber-difficult feelings. They also have a sense of humor about it, which is nice for us readers; a lot of the couple's conversations consist solely of bummers, so their jokes help keep everything from feeling too heavy.
Most of the language in the book is modern and straightforward, but it also borrows passages from literary writers that have reps for being difficult—Virginia Woolf and all manner of Russian novelists and poets whom Finch and Violet like to quote to one another.
Don't worry, they aren't that pretentious; they're just really feeling those lines. And hey: They also quote a lot of Dr. Seuss.