Emotions aren't a choice, but emotional awareness and control can be. When annoyed or upset, stopping to ask yourself why you're annoyed or upset can be very beneficial. The old "stop, take a deep breath, and count to 10" advice from grade school has some merit. (You don't literally have to stop, take a deep breath, and count to ten. But making a conscious effort to be aware of one's emotions is a worthwhile exercise.)
I've found that literally stopping to take a breath is actually a very good practice for this situation. Helps me avoid a lot of bad moods caused by inconsequential things. Process the negative information, realize it's inconsequential, and return to a state of calm.
(I don't count to ten)
This applies for small things, but most things that people get distressed about are small.
It's amazing how well this works, at least for me.
Sometimes I even make an effort to think to myself, "How would X respond to this situation?" Replace "X" with anyone of your choice who would probably be a lot cooler than you under stress: Jesus, Spock, Batman, James Bond, Yoda, Mr. Rogers, or whoever the heck epitomizes "chill" in your worldview.
It sounds cheesy, and it probably is. But it works. Chances are, the "X" of your choice wouldn't react hastily or let small things get to him. Shit happens to everybody, but stopping in mid-emotional-flareup to process an appropriate response is a skill. And skills can be developed.
