Why do we get angry?

It's part of the "fight or flight" sympathetic nervous system. It is an evolutionary leftover from a time long ago when we really needed to muster extra energy, and quickly, to either fight for our survival against nasty carnivores or run away. There are perhaps times when it is still appropriate (maybe to motivate someone to take on injustice) but largely it is instinctual and involuntary. How we act on our anger is a different story. That is always a choice.

Anger is used as a defense mechanism when we feel threatened in any way.

Because we cared for something that disappointed us by not satisfying our expectations of them that resulted in self anger for trusting that person which we then reflected on our anger on that person