one theory is that... the emotional expressiveness / reactivity is due to an 'invalidating environment'.

when the match between the individual and environment is poor then often people have to magnify or act out a little in order to have their emotional needs met.

oftentimes there is trauma too... hence heightened startle reflex / reactivity of emotional expression.

sometimes individuals don't really learn how to regulate their emotions because parents didn't know how to regulate theirs...