In my daily life I interact with non Jews, like many of you. Around Yom Kippur, I'm never reluctant to tell them how we celebrate the holiday. I mention the fasting. I tell them we spend hours in shul. I explain that the day has a unique power and an enduring hold over my emotions and imagination. And I do this even though I know they can't relate. In fact, I'll often preface my explanations with an "I know this is strange, but.."
Sukkos is different. I don't think I've ever volunteered that we gesticulate with fruit. This is not because I think one strange set of rituals is less strange than the other, but because I know the gentiles can relate to fasting and prayer. They and their holy people do that, too. But fruit waving? Too weird. Why would anyone do such a thing?
I'm aware of two Torah-true explanations, neither of which I much like:
(1) There are all sorts of magical, kabalistic forces and energies in the air, especially at this auspicious time of year. The arbah minim are a sort of antenna that allows us to access, and perhaps collect those invisible waves. Why I hate this explanation: a) Taking the arbah minim is a biblical commandment, but the bible provides no hint at all that such forces and powers exist. The Jews didn't start to think about magical forces until much later in our history. b) As the Ramban famously said, "He who wishes to deceive calls his witnesses from far away." We have no evidence at all that such forces exist. Asserting that these invisible, imperceptible forces are the reason we lulav wave, seems to much like someone is pretending to knowledge he doesn't have.
(2) Waving fruit reminds us of Israel and the harvest and makes us happy. (Rambam) Why I hate this explanation: Really? Waving fruit makes us happy? Waving fruit?
So what's the deal? I mean the real deal. Why did Jews start waving around the Arbah Minim. Because even if you want to argue that God's original command was to take the Arbah Minim and build a sukka with them, at some moment in time, the Jews got a different idea in their heads and chose to start waving the things instead. Why would we do that?
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