Lord of the Flies is a novel that has stood the test of time for many reasons. It contains characters who are both relatable and multi-faceted, and it forces readers to examine their own hearts and minds as they follow the trials and tribulations of the boys stranded on the island.
We can see bits and pieces of ourselves in the characters. Some of these things we see may make us feel uncomfortable, or even ashamed. The social dynamic of the group of boys is both unique and recognizable. While very few of us have experienced being stranded an a deserted island, we have all been confronted with situations in our lives that have forced us to take on roles we may or may not have been prepared to assume. When faced with adversity, groups of people tend to gravitate or be pulled into specific roles in order to achieve a common goal. The necessity for humans to analyze a situation and then work together to solve a central problem is universal.
I think that an appropriate thesis statement for this book would be something to the effect of: "Lord of the Flies has retained its relevance throughout the years because of its ability to expose and explore both human fragility and human resilience." Or perhaps: "Lord of the Flies remains as relevant today as it was upon its release in 1954 because of the way in which the author uses central characters to show universal human emotions and behaviors when those humans are faced with adversity and peril." You can use either statement as a starting point and go on to analyze and critique different characters or situations using your own thoughts and feelings.