Okay, this one is an easy one, but it takes some paragraphs to explain. Your friend is right that there are causal explanations for things biological and social, but is getting the causality all wonky. We don’t hunger because food exists. We hunger because our life processes cause us to expend energy, and not being plants we need to eat. The food doesn’t cause the hunger - we’d be hungry on the moon - but we’ve learned throughout evolutionary history that our life processes need energy and nutrients, and we’ve made the evolutionary decision to get them by eating. The same is true for water and sex. Hunger exists because otherwise we wouldn’t eat, and horniness exists because otherwise we wouldn’t reproduce. Look throughout the animal kingdom - does it make any sense to say that a spider or termite gets “horny” on any way that remotely resembles a human logging onto pornhub? There is indeed causality, but the presented argument is flipping it on its head.

Animals sense and react to their environment. It’s a survival thing. It’s baked into our DNA and has been for untold millions of years. One aspect of this is a feature some people call the hyperactive agency detector. It is our tendency to attribute some event - a thing that goes bump in the night, for example - to a conscious being. An agent. It might just be your door blowing open in a gust of wind or a badly placed book falling to the floor, but chances are that you will have a startle reaction and at the very least listen intently for a couple of minutes with a pounding heart. There’s also a good chance that it might get your dog barking or your cat to sit up and listen. It’s the default assumption that an effect was caused by an agent rather than some impersonal environment action.

You can see how that’s beneficial to survival. If you hear a twig snap while you’re sleeping in the jungle, you’re more likely to survive if you make some preparations than if you just ignore it, because it may actually be a tiger. We can get into how much energy is wasted on a false positive versus the risks incurred by a false negative, but you probably get the idea. If something happens, our brains tend to assume that somebody did it.

So when the human conceptual scope expands such that “something that happens” includes things like weather and earthquakes and crop failures and wars won or lost, we also tend to attribute those things to an agent. We need an agent considerably bigger than a tiger - or even a fellow human - if we’re talking about things on that scale. Our hyperactive agency detector ends up interpreting causality and agency where none exists.

Anyway, that’s god. There are also billions of people who have no “yearning” for god. There are religions that are largely if not entirely atheistic, but which still seek to understand the world through causality. There’s also a drive - this one is unique to humans as far as we know - to create cohesive belief systems that help to unify societies. The third piece is our evolutionary tendency to believe what people we trust tell us, especially as children.