Being completely incapable of not responding when someone says they'd love to hear what I have to say, and guilty of assuming that Julie was speaking for everybody, I decided I should expand on the topic here.
Julie has highlighted the distinction between antagonists and the source of conflict by pointing out that some stories don't (or can't) embody the forces working in opposition to the protagonist in a single character. Even so, those stories still must have a source of conflict, whether internal or external.
If you ask writers about kinds of stories you'll likely get a variation on the classic triumvirate of man vs. self, man vs. man, and man vs. nature. I like to add a few more gradations to the sources of conflict:
- Self - Internal demons, conflicting needs or desires, psychological dissonance
- People - Lovers, family, friends (i.e., people with whom the protagonist has more than casual relationships)
- Society - Organizations, clubs, cabals, conspiracies, churches, companies, bureaucracies, armies, parties, governments, movements, etc.
- Nature - A particular feature of the natural world: animals, mountains, oceans, storms, droughts, etc.
- Universe/God - The external world in general
But there's a deeper reason that the conflict in the vast majority of stories occurs at the level of people and society: conflict is fundamentally interpersonal.
Before you accuse me of forgetting the question, let me explain: in the same vein as the philosophic question about trees falling in the forest, there are no stories about the world that existed before people. It's not that things didn't happen--indeed, if contemporary CGI-rich dinosaur documentaries are to be believed, there was plenty of red-in-tooth-and-claw conflict--it's that there was no one around to attribute significance to the actors and events. Was it good or bad that the tyrannosaurus took out the ailing duckbill?
Often, scarcity is the source of conflict. A great many sports, for example, depend on the fact that there are two teams and only one ball. But the significance of the conflict depends upon the meaning we assign to it.
"Okay," you say, "what about a man trying to conquer a mountain?"
It all depends on why he's trying to conquer the mountain. If he's trying to get to the other side to find the cure for the fever in his village, then it's a heroic conflict. If he's trying to get to the other side to enslave the village there, then it's a very different sort of conflict.
Put another way, regardless of the source of conflict, the first thing readers want to know is, "Why should we care?" Most people find it very difficult to care about anything unless they can do so in personal terms. When another person or persons oppose the protagonist, readers immediately recognize the personal stakes. When the source of conflict is non-personal--either internal or external--you must show why that conflict matters to your protagonist and, by extension, your readers.
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