It’s hard because any offense concerning another person’s identity, no matter how ostensibly trivial or unintentional, inevitably becomes a proxy for the debate over who is and isn’t defined as a full human being. When people feel (justly) that they have been historically excluded from economic and political and civic life on the basis of their identity — and therefore, within the context of a humanist liberal society, excluded from humanity itself — anything that conjures the slightest whiff of such exclusion becomes grounds for rage.
It’s hard because movements for social justice are by definition concerned with calling out exploitation, and therefore anything that questions the legitimacy of *any* given grievance is perceived as giving aid to oppressors.
It’s hard because social justice, at least as we’ve known it, has typically consisted of challenging the legitimacy of authorities’ adjudication of social norms, and thus any new attempt at adjudication (ie, this thing was legitimately offensive while that was not) is automatically suspect.
It’s hard because while humans are often remarkably good at dealing with complexity, nuance and gray areas in their personal lives, social justice inherently involves collective action, which requires bright lines and black-and-white definitions.
It’s hard because skepticism is inherently difficult, and therefore in a time in which all values and norms are in flux, the best tend to lack all conviction and the worst tend to be full of passionate intensity, etc. It’s hard because a degraded, shallow consumer culture is barren soil for new manners and customs to take root.