One possible thematic statement that encompasses your quote from Eye of the Needle could be: "the complexity of peacetime can be unsettling and overwhelming, leading individuals to cling to the simplicity and purpose provided by conflict." This theme suggests that in times of war, individuals may feel a sense of clarity, purpose, and direction in their actions and motivations. This is because they are driven by a clear-cut enmity (positive, active, and typically mutual hatred or ill will) toward the enemy. However, when faced with the prospect of peace, they may feel uncertain and confused about how to navigate the complexities of civilian life and may therefore be reluctant to let go of the structure and purpose provided by the war.

In simpler words, sometimes when people are in a war or conflict they know exactly why they are fighting and what they need to do. It can be easier to make decisions and feel motivated when there is a clear enemy to oppose. But when the conflict ends and peace comes, things can become more complicated. People might not know what to do with themselves or how to deal with their feelings. It can be scary to face new challenges and make decisions about the future. That's why some people might be hesitant to let go of the structure and purpose that the conflict provided, even though it might be better for everyone in the long run.