For all kinds of reasons people may anger us. Often, though, once we get underneath the reason for anger, we will find compassion and that's a much better place to be.
Compassion does not mean that you have to withstand abuse or disrespect, but instead, it means that you can counter the events that anger you, with a compassionate response.
For example, I remember a time when a sick friend was not kind to me. I was angry and had trouble being compassionate. I could not understand why the friend was acting with such contempt and hurt, but now, looking back, I wish I had found my way to compassion because clearly that friend was dealing with a lot of struggle at the time--a struggle that was difficult for me to understand at the time.
To find our way from anger to compassion is the better path to take, and we may need the support of others and some research to get there, but it's worth the journey.