There is truth behind your question, but unfortunately there are many, many causes of anger in addition to unfulfilled expectations. There are many perceived shortcomings in the treatment by others, as well as the vicissitudes of life in general, that impose limitations or create a state of lack in some respect, leaving a person unsatisfied and unhappy. Whether anger arises is a function of the level of vibration the person has cultivated for their general mood state at a given moment and the ease in which they can be, in effect, pushed out of this vibration for it to diminish and render them more and more susceptible to darker emotions that are of a lower vibrational nature. We are using this terminology not because it is metaphysical but because it is functionally accurate that vibration is a continuum, and emotions lie along this vibrational continuum as an indicator and often a motivation to take action in some way, hopefully to restore a better inner alignment with a loving vibration and not taken as an impetus to lash out at someone or cause a rash act that might later be regretted in response to inner dissatisfaction.
A major category of cause behind feelings of anger, we would state, as a broader category of unfairness. Being treated unfairly in some way covers a lot of ground, whether done by individuals to a person or by institutions, or a combination of players where people who represent an institution are the deliverer of bad news in some way that invokes the perception of unfair criticism or treatment. And the emotional consequences will say a lot about the interior functioning of the individual and in some ways their relative wellbeing, whether they are roused into anger or simply able to respond with logic and reason in the assumption that their inner truth and wisdom will carry the day and there is no need for ruffled feathers. But some will take umbrage and resent an implication they are faulty or unworthy in some respect, or simply being treated shabbily. This may come with a high degree of justification when there is true unfair treatment on display or it might be wounds to the ego, as happens when people are less complete in the sense of being truly in divine alignment and have only their own ego to cling to for self‑worth, and that is a precarious position to be in because almost any negative assessment of them will be seen as a threat, and the lack of interior alignment gives them little to work with, except to return negativity in kind or perhaps magnified through anger. In contrast, often a person with fewer inner resources of strength will simply be wounded and may passively accept their unfair treatment because they feel helpless, even though wronged, to do anything, to fight back and defend themselves, and potentially enter into a helpful negotiation to right the wrong in some way. Some who are quite beaten down and lack a belief in themselves and their own worthiness will accept unfair treatment as simply further evidence of their decrepit state and will surrender to almost anything negative sent their way.
We see both anger and abject acceptance of unfair treatment as unhealthy and a sign there is a significant healing need and opportunity on display where inner growth can be achieved through prayer work and healing requests. All too often people with an angry disposition are simply seen as someone with that personality and assume nothing can be done. This will usually be the personal perspective of such an individual, and their emotions provide all the justification they need for reacting harshly to unfair treatment. Even though many others would experience no anger whatsoever, many times there are misunderstandings and people are neglected in a way that is not at all personal or intended, and yet the anger-based individual will explode and lash out quite readily and perhaps then create animosity in their would-be opponent and create an enemy in the process. As with everything in life, healing starts at home; there must be a personal inner awareness and resolve to seek something better before anything can happen, including divine intervention on a person’s behalf. So it is good to have these discussions to put things in perspective, to not simply accept the world as it seems to be and oneself as you seem to have been made, but know you are, inevitably, a work in progress and incomplete and there are ways to grow through learning and gaining of wisdom.