This sounds like a question in psychology (study of the mind) or about neurology (study of the brain). Possibly also sociology too (study of groups of people).
However, I think any of those studies and fields of study would help explain how we get frustrated more then why we get frustrated. So I don't know if why we get frustrated has a good means of study, because people get frustrated over different things. That would make it difficult to gather answers for why one person gets frustrated that can also be true for the same reasons why someone else also gets frustrated.
_________________________________
Looking at the question of frustration from a practical perspective (based on our own observations), instead of a scientific perspective (something to test and experiment), my thought for why we get frustrated is because things don't go as we planned or expected and we weren't prepared for them.
After that though I don't think it's science of why, as much as a matter experience, attitude, and how you were raised or trained to handle situations that account for why some people get frustrated more easily, and others are better at rolling with the punches.