They fit the book, they fit my mood, they evoke the image of that story to me. It might be where I first read that book, or what I felt I needed to make the story complete to me.
A few of my examples...
A Superior Death by Nevada Barr - A glass of white wine while sitting on a lounger on a sun-dappled porch
Good in Bed by Jennifer Weiner - A mug of chai while curled on an overstuffed sofa
Anything by Enid Blyton or J.K. Rowling - a cup of Bovril while reclining on a long couch
The Da Vinci Code by Dan Brown - a grande coffee in a quiet coffeeshop
Memoirs of a Geisha by Arthur Golden - a cup of green tea while lying on a bed overflowing with pillows
Driver #8 by Dale Earnhardt Jr or A Little Bit Sideways by Scott Huler - a bottle of Bud while kicked back on a quiet front porch
Anything by Meg Cabot - a Diet Coke and a quiet corner where giggling is encouraged
Bridget Jones's Diary by Helen Fielding - an iced tea while sprawled on a blanket in the grass
The Probable Future by Alice Hoffman - a mug of hot chocolate while watching the rain pour down from a comfortable loveseat
The Time Traveler's Wife by Audrey Niffenegger - a steamy cup of tea while staying up late in bed reading
Does everyone have such tastes, such settings, such memories of their favorite books?