Contemplating a male exemplar of the human species who decided to witness an event that is able to prompt him to be angry or upset,  the following can be observed:
 

His face gets as red as this screen. His blood pressure skyrockets. All systems are on overload. (All of this is very unhealthy, of course.) Then his increasingly irrational emotions are making him more and more unhappy. He starts getting more and more aggressive and may begin to hurt others, especially those around him that he loves or that he wants to love him.

Why, then, did he do what he did?

There is no 'logical' explanation for this behavior no matter how clever the justifications for it may be. It is part of the 'case' of a person. Which is that in-sane part of man that is at the root of all his misery.

For some people it appears to be just a habit that they acquired from others, like their parents. Others think they need it for their system every once a while. Some get rarely upset; a very, very few once every quarter century.

In any case, again, he hurts himself with his upset and anger.
 

So why doesn't he stop hurting himself?

To begin with, he rarely recognizes that he is punishing himself more than anyone else.

But even then, in order to hide his own evil intentions or past aggressive acts or failures to help, he pretends the 'reason' for his upset is whatever he decided to experience. Which is what he thinks that happened 'to' him.

Since he knows deep inside that the pain coming with his anger and upset is foremost his own problem, he needs to continue to justify every upset in the future in order to remain 'right' and not 'wrong'. This way, the upset persists in time and may even escalate. In any case, it can be triggered again at any given time in the future unless he would something about it.

How is it working?

In brief words, an upset or anger is a copied behavior pattern because of a past  failure and is commonly accompagnied by an evil intention to harm someone. (The Who/What/When/Where procedure will always disclose the exact mechanism at work).

In short, anything that reminds him of his own unwanted conditions, past or  present, can serve as a trigger to go into this self- (and other) punishment mode.

But if someone did wrong to him, isn't that a good reason to be upset?

There are two possible (re)actions if someone did something wrong:
 

• fight back, or
• lean back and tolerate it.

Even if this would not be a circumstance that can be observed every day and everywhere, there is still another little factlet:
 

Everybody determines his own fate.
 

"But one can't just simply change one's emotions..."

One can sit down and find one's own intentions and past actions that were triggered by the 'reason' of the upset and, once found, dissolve them. This is easier said than done and may require assistance (at least in the beginning) by a skilled practitioner.

In the meantime, it very often helps to just look at similar outbreaks in the past and compare it to the present situation.

If there is some experience already there with 'running processes' by oneself, one can try to look closely at what had happened that was triggered again and find the 'who/what/when & where' as well as any plan, decision or precept that were done in the wake of the incident.

In any case, the sheer existence of the tiniest bit of rage or upset in one's mind should be more than ample reason to read on in this marvelous piece of under- and overground literature:

the space-  and timeless

'Little Purple Notebook On How To Escape From This Universe'

Hmm, if you should really think you would be an ape, how about getting yourself a banana?