That is a question that dominant groups always seem to ask subordinate groups when the subordinate group question their subordination. It is a question that implies that the people fighting for their rights do not have a legitimate complaint, otherwise why the need to question the source of their anger. It is also the question one would ask of a child thus reducing the subordinate group to the level of petulant children. It is a question that comes from a place of unacknowledged power and privilege, a place that cannot understand why anybody would be unhappy with the status quo. That the status quo disproportionately favors the dominant group over all others is considered just the natural order of things.
Of course is isn’t the natural order, it is the result of human actions designed to favor their group over all others and that is why people outside of the dominate group are angry. They can see that the status quo is a artificial construction that can be changed to make things better for them. They came by their anger legitimately, I can by my anger legitimately, so don’t you dare question it.