The speaker is hurt and baffled by how people treat each other. Again and again the poem asks the same question, "what is it with people?", and each time adds another example of cruelty: people who pull us down when we're happy, forget our good deeds but remember our small mistakes, and spoil our victories. The speaker also points to hypocrisy, describing faces that look sincere at the front door and then spill lies and hatred in the back alleys.
The mood is bitter and weary, with real anger in images like "rubbing the salt in our wounds." But the last stanza shifts. The speaker suggests these same people "cry in their sleep" and are full of jealousy, secretly begging for happiness. The anger softens into something closer to puzzled sympathy. The title, "to bear a mask," fits this idea: the cruelty may be a disguise for pain, and everyone may be hiding something.