A recent New York Times article called "The Next Big Thing in English" reviews what makes fiction interesting to us from an evolutionary psychology perspective. I especially like the comments of Lisa Zunshine, a professor of English at the University of Kentucky.
"Ms. Zunshine is particularly interested in what cognitive scientists call the theory of mind, which involves one person’s ability to interpret another person’s mental state and to pinpoint the source of a particular piece of information in order to assess its validity. (...)
"Humans can comfortably keep track of three different mental states at a time, Ms. Zunshine said. For example, the proposition “Peter said that Paul believed that Mary liked chocolate” is not too hard to follow. (...)
"Perhaps the human facility with three levels is related to the intrigues of sexual mating, Ms. Zunshine suggested. Do I think he is attracted to her or me? Whatever the root cause, Ms. Zunshine argues, people find the interaction of three minds compelling. “If I have some ideological agenda,” she said, “I would try to construct a narrative that involved a triangularization of minds, because that is something we find particularly satisfying.”
In other words, my story will be inherently more interesting to the reader if, rather than just a conflict between two characters A and B, I have the interplay of a third, C. Filtering the conflict between A and B through the opinions of C will not only enrich the situation, it will make it more interesting to the reader because we have evolved to be absorbed by events involving different people's perceptions.
That A says B is a jerk, and B says A is naive, is made more interesting by each expressing their views to C (somewhat differently from the way they express their opinions to each other, which interests us because it helps us assess their credibility).
C's replies are also interesting, because C's prior involvement with each will also color the responses, and again force us to think of issues of credibility and personal histories.
This is not just a richer description of the conflict in an objective way. It draws our attention like a flashing light, because we have evolved to be interested in - especially - the interaction of three minds.
Hence the Eternal Triangle. But it applies to all conflicts in all stories, whether romance is involved or not. It appears to be fundamental to human thinking and to what interests us.