People get angry for all sorts of reasons, of which, sometimes that anger is absolutely justified and other times it really is not. Additionally, just because your anger is justified, does not mean that you can simply fly off the handle, for that would often be considered an overreaction, but instead, it is important to keep your anger in perspective, and if possible, to try to understand what triggered that anger and why.
For instance, anger can often be activated by having your self-will denied by someone else that has authority over you, or various other reasons in which you know that retaliation is either not an option, or isn't going to work in that situation. You also can get angry, when people don't behave in a manner that you desire for them to behave, especially when you feel that they should listen to you or that they owe you the right behavior but are not behaving correctly towards you. Additionally, you get angry, for being denied, fairness and justness that, clearly, even when you have subsequently calmed down, really was unfair and unjust.
In many instances, in many different ways, anger is a reaction to being placed within a situation in which you do not have the power to overcome or to change what is angering you, so that if you are driving, on a one-lane road and the person in front of you, is driving like Miss Daisy, you may easily feel the anger well up inside of you, especially when you can see nothing but open blacktop in front of her vehicle. So too, when you drop something of value such as your cell phone onto the cement and the screen cracks, you will get angry, at yourself for dropping it, at the expense of fixing it, and the frustration of something bad happening, that you can't undo.
In many ways, to get angry, isn't really the problem, for those people that never get angry, are either saints in some sense or another, or so absolutely passive or withdrawn in their demeanor, that they aren't really here in the now. The problem with anger, is then, more times than not, the inability to temper that anger with the knowledge that you really don't have the power at that time and space, to do anything about the situation that has upset you, so that, the best thing to do is to somehow, place these things into their appropriate perspective, so that understanding that the power to change what is frustrating you is actually not yours, to then make a determination to try to deal with the situation in a way that recognizes that, which should help to keep you calmer.
Nobody likes it when their will is denied, but it's important to recognize, that in any civilization, that your will is just one will amongst all the others that are part of that civilization, so that, everybody in one form of another, is having their will denied, from time-to-time, just as you are, which signifies, that the best way to deal with this, is to acknowledge that you cannot always get your way. Then, in recognition of this, understand well, also, that you are not all powerful, which means that frustration and anger will occur within your life, in which, your desire should be not necessarily to never get angry, but to demonstrate that you are the master of your anger, by placing such anger into its proper perspective, for to become exceedingly angry about something that you cannot change, does nothing to resolve it, so then, you are better off, just recognizing, that this too, will pass.