I'd define anger as the response toward an event for which the result matters, the result of which goes against your expectations, and in which you have invested what you take to have been sufficient and necessary resources for it to have gone as expected. Frustration is closely related but is not quite the same thing. Frustration has more to do with an inability to control your current state, as in to end something unpleasant. Frustration can be a secondary cause of anger, especially if you believe that you ought to be able to control the situation given past experience and knowledge/competence: take hitting your thumb with a hammer despite having driven 25000 nails in the past. I think we are angry because we have a culture which across many generations has amassed the knowledge, skills, technology and moral structures to cause us to expect something in our world which we are not getting. We are watching our culture start to question assumptions as basic as gender. We know we are way beyond this. It causes anger. Moreover, we are frustrated because we feel in some sense unable to control this situation, despite feeling that we have the means to control it theoretically. Now we have identified the reasons why, and still feel stifled by the ignorance and antagonism of our own people who either refuse to see our enemy or merely refuse to look.