Angry is a habit
It's easy to imagine habits like a scotch after dinner, biting your nails or saying, "you know" after every sentence. An event or a time of day triggers us, and we go with the habit. It's easier than exploring new territory--it's merely a thoughtless response to an incoming trigger.
But emotions can become habits as well.
Distrustful is a habit.
Lonely is a habit.
Generous is a habit.
When that stranger doesn't do what you expect, is your response to assume that she's out to get you, trying to make an extra buck, looking for a shortcut? Or do you default to the habit of giving that new person a chance to explain herself?
Habits are great when they help us get what we want. Bad habits, on the other hand, are bad because the shortcut that satisfies us in the moment gets in the way of our long term goals.
Once you can see that your emotions are as much as a habit as cracking your knuckles, they're a lot easier to work with.