Fear
Green Town may be charming but it's not perfect. There is dark, unfathomable danger in the ravine and in the figure of the Lonely One, a man who preys on women and strangles and kills them. The ravine is perhaps more terrifying because its dangers are more nebulous, more difficult to pin down. On the evening when Douglas does not come home from playing with his friends, Mother palpably embodies the fear that humans have of the wild unknown of nature as well as the interstitial places where nature butts up uncomfortably to civilization. Fear also affects characters in more existential ways, such as when Douglas realizes he will die one day and consequently becomes sick and depressed. The only way to combat fear is to accept what one cannot change, and to live life pleasurably and rationally.
Technology and Modernization
Technology and modernization are some of the central themes of the novel and resurface quite often. While technology is important and Bradbury is not eschewing its value completely, it is clear that technology is no replacement for human relations. One of the most important moments when Bradbury asserts this is when Grandfather tells Bill to appreciate the time spent gardening and mowing the lawn rather than embrace technology that precludes someone from spending time outdoors. Similarly, technology and modernization can also strip away the charm and beauty of life, something Douglas notes when he ruminates on how the trolley will not sound the same and won't have the same memorable sparks and bells.
Nostalgia
Bradbury clearly delights in the past. His Green Town of 1928 is a fully-realized world, carefully and lovingly drawn and chock-full of details about what life was like then and there. It is a nostalgic work, perhaps even a sentimental one. However, Bradbury doesn't embrace nostalgia fully. He once said, "A man cannot possibly speak futures unless he has a strong sense of the past." Looking at the past, then, helps us figure out what we want the future to be. He doesn't lament the loss of a golden age but presents it with all of its nuances to encourage readers to appreciate every moment that they're in and actively work to make the present and future what they want it to be.
Coming of Age
Though the novel spans only a couple months, Douglas clearly experiences a coming of age. At the beginning of the summer he undergoes a rite of passage in the forest when he realizes that he is alive and must soak up everything life has to offer. He then spends the summer trying to negotiate what this means, which proves psychologically destabilizing when he confronts the loss and deaths of friends as well as the knowledge that he too will die someday. With the help of Mr. Jonas and his bottles of precious, magical air he decides that he will remain living and pass on what he has learned about the beauty of life in all of its incomprehensibility and pain and confusion to others. Douglas comes out of the summer both (slightly) older and (much) wiser.
Friendship
Bradbury extols the merits of friendship in this work, but it's not just the Douglas-and-John-Huff type of classic childhood friendships we all have/had. Rather, friendship in this work crosses generations and backgrounds. Trolley operators take children on picnics, old men and women wax poetic about their younger days, and the porch becomes a place for everyone to gather, talk, and just be. Friendships are more important than money, technology, or power; friendships are what sustain people when life is difficult.
Memory
There is pleasure in the present moment but Bradbury impresses upon the reader the significance and power of memory. The bottles of dandelion wine contain within them the memories of the summer and thus those warm memories can be accessed during the cold winter. People possess memories and can share them with others; Colonel Freeleigh is labeled a "time machine" for how effectively he's able to transport people into the past. The connection to the past is what imbues the present with meaning.
Time and Change
Douglas has to come to terms with the fact that change isn't always good and time can betray him. In regards to the former, he is distressed that his friend move away, fun things like the Green Machine and trolley vanish, and people he knows die. He concludes that he cannot depend on either people or things. As for the latter, Douglas tries his best to slow down time, both in regards to the summer itself and to specific moments (as when John is going to be moving away and he and Douglas want to stretch out their afternoon together). Ultimately he has to learn that he cannot control everything; time will pass and things will change and part of growing up is coming to terms with that.
