"Stress reactivity occurs specifically in situations that diminish one’s control and where the prospect of being negatively evaluated, rejected, and/or shamed are contextually manipulated (Dickerson et al., 2004)."
I find I get the most stressed when I'm excited about a new challenge -- a new work project, seeing a path to achieving a goal I've been working towards for a while. When I start to feel too excited (or in the framework of this paper, I start to expend too much energy and my heart rate is elevated thinking about all I want to do to overcome the challenge), I can generally reign in the stress by reminding myself that failure happens, my peers will understand it, and I don't have complete control over anything anyway.