Read a study recently where the researchers discovered that folks with a diagnosis of Borderline Personality Disorder are markedly faster in their ability to discern subtle affective change in facial expressions than the general population. This data from the study also suggests that this population of people DO NOT make more mistakes in assessing mood than the general population.
Why would some folks be more sensitive to very subtle affect changes in others? The study didn’t attempt to answer that but I suspect it is because they (a) needed to be vigilant to potential danger, (b) they themselves are highly emotional, and (c) they have been “schooled” to believe that others are more right in their assessments and so they should accept other people’s feelings as more true.
Musings of a Christian Psychologist 
Can you provide the citation for that study? Thanks!
The kids with BPD that I’ve worked with seem to have exceptionally attuned “radar”, usually for the purpose of reading the people in their surroundings, then acting out in such a way as to orchestrate a situation that justifies those “self-fulfilling prophecies” they carry around with them. Helping a BPD kid recognize those patterns and cycles is akin to stepping on their life-giving “air hose” and cutting off the oxygen to their soul! Soul survival seems to depend upon exonerating and strengthening those wacked schemas…and hypersensitive, albeit goofed up, radar is a prime tool. NASA should have something so sensitive! We could detect stickpins fired at thousands of miles away with that kind of technology!
Katie,
sorry, I meant to include this:
Lynch, T.R. et al (2006). Heightened sensitivity to facial expressions of emotion in borderline personality disorder. Emotion, 6(4), 647-655.