So, a "sunk cost" is, basically, a cost that's already been paid. in comparison a prospective cost is one that's coming up. In the classic examples given for a sunk-cost fallacy, like the decision to maintain an older car or dump it for a new one; the sunk cost is all the past repairs and the initial purchase price. There's also all the realized benefits or profit on that side of the equation.

The generic fallacy is that people generally are influenced by those sunk costs. We generally continue something, or rather, are more likely to continue something we've been invested in- time, money, effort. Emotions. Where this fallacy leads us astray is that when those sunk costs are accrued, they can't be recovered. They have no bearing on the results of current decisions.

in the context of relationships, people change, and relationships between people change. We grow, maybe together. maybe we grow apart. One thing that will certainly not be happening is going back to what was.

and while it's perhaps churlish to discuss relationships in terms of economics... those costs and rewards have already been accrued. The question at hand isn't whether or not the relationship was worth it, but rather, is the relationship still worth it, and will it be in the future. if you suffer the sunk cost fallacy, you're basing your decision on what has been, and that's almost always a great way to get bit in the ass.