What is the theme in Killing Floor by Lee Child?

Love and Marriage vs Freedom and Anonymity is a theme in this story. Jack Reacher is pleasantly surprised by the depth of his feelings for Roscoe. Though less than a week passes from when they first meet, and their introduction doesn't place Jack in a very favorable light, she falls for him too. Together they begin to investigate the murders and their irregularities. The further involved they become, the more Jack finds that overcoming fear and adversity has more purpose with Roscoe in his life. When they are separated, even briefly, he can't stop thinking about her, and looking forward to the next time he sees her.