It is said that behind anger are some other emotions: for example, "...We feel hurt, frustrated, threatened, judged, wronged or disrespected....." which is true.
But these emotions aren't the original cause of anger. Behind these emotions there is some believe, need, desire, the self-image and pride. So behind the emotions of fear, shame, frustration, disappointment, etc. there is always a deeper cause. i.e. issues of low esteem, inferiority, insecurity, need to control things and persons, greed, some desire, attachment with certain forms, a tamasic or rajasic state of mind, etc.
Although many things can be related to anger the immediate cause of anger is desire. When a desire or what we want is not fulfilled desire transforms (like milk to yogurt) to anger. Desire and anger are the same energy. The desire mechanism function also in relation to all other psychological tendencies and needs as for example that of pride, self-image, the need to be recognized or accepted etc.
Second. Anger is always expressed against someone or something, (even against our selves), that impede us to fulfill a desire, a wanting, a need (for example, the need to feel worthy, sufficient, special, recognized, safe, accepted, respected etc.).
Anger in reality is an attack to others in order to tell them forcefully that we don't like and we don't want them to obstruct our plans, our desires or to reduce our self-esteem, self-image etc., as well as that we will stop them (him /her) to do so, and according the circumstances we can also threaten them that we will create problem to them as they did to us and also that we will make them suffer like us.
Anger is a primitive emotion that affects negatively our health, vital energy, feelings, our mental clarity, discernment and reason, causes enmity and separation and make us act destructively and lose our capacity to do our best in the situation we are engaged.
Anger is one of the strongest psychological tendencies that destroys relations, harmony, peace, unity, happiness.