Drawing Parallels Between Finance and Psychology: A Psychology Metaphor
Classically, we might call emotional volatility neuroticism.  We could also refer to a continuum of affective reactivity (high vs. low reactivity; e.g., Gross, Sutton, &amp; Ketelaar, 1998). 

Emotion dysregulation need not imply emotional volatility. For example, depressed patients show stable depressed mood or emotional inertia (Brose et al., 2014), which is related to impaired affective updating under stress (Pe et al., 2015) and perseverative thinking (Nolen-Hoeksema, Wisco, &amp; Lyubomirsky, 2008).

A lot of these ideas can be pushed under the umbrella of affective dynamics, or how affect/emotions change over time (for recent theory, see this special issue of Emotion Review, 2015).

We can also say that fluctuations in affect represent deviations from allostasis (Sterling, 2013), which motivate us to engage in self-regulatory behaviors that bring us back to allostasis (Inzlicht, Bartholow, &amp; Hirsch, 2015).  

As part of a predictive regulation framework, emotional instability may result from feedback dynamics between positive and negative predictions about the world (Eldar et al., 2015; see the image below).  Relatedly, maladaptive priors and associated attentional biases (i.e., inappropriately weighted prediction errors) may also account for volatility (Huys, Daw, &amp; Dayan, 2015).