This has always been a topic of interest to me because I've never heard or read an explanation that I believe. I've heard that your body is regulated by your brain, therefore, you have a physical reaction to stress. Really? Stress isn't a real thing, right? I mean, you can't buy it, or touch it or taste it - so how is it real? Stress is something you create by how you react to any given situation or scenario. Isn't that what ALL feelings are?
I figure that your body has a much more powerful physical response to negative feelings because of the "fight or flight" reaction to unwanted situations. Think about this: when you're sad, anxious, heartbroken, frustrated or overwhelmed, many people feel it by getting a stomach ache, sleeplessness, increased heart rate, headaches, etc., etc., etc. On the other hand, when you get good news, are happy and content, excited and motivated, you don't feel any better than you did before. You may smile and your voice will reflect that you are upbeat, but your body will not physically feel any better than it did before you got those positive feels. Otherwise, we'd have no headaches. If you had one, you could ask someone to tell you something exciting and happy and your headache would magically disappear.
That’s the part that screws up the "your brain controls how your body feels" explanation. If it did - it would and should work both ways.
In theory, if we could better control our reactions to undesirable situations, we wouldn't feel bad when we're upset - the key is not to let things upset us. (The humor in that statement is not lost on me.)