Good question. If found a good article: https://www.masterclass.com/articles/the-main-types-of-antagonists

And a good example - the sea in 'Robinson Crusoe" is the antagonist in that story.

So in my mind, perhaps could be anything, even any feeling, that impels the characters to act in a certain way. Greed, jealousy, anger, are some feelings I think of that could cause people to act in ways that aren't quite so pleasant. And then explore why the character has those feelings. What caused the intense feeling? Why did the protagonist do the action? Was it greed that prevented them from coming together? Anger? Jealousy?

That is the first thought that came to mind as I read your email.

Good luck,
NormaJean