In "We Wear the Mask," people suffer because life is painful and people hide their pain from the world. Dunbar writes the "clay is vile" beneath their feet, implying life on earth is hard and difficult. They do not change their situations because they cannot -- it is implied that to live is to suffer. In the poem these people appeal to Christ to save them.
In "Sympathy," the bird suffers because it is caged. It sees the beauty of the world around it but it cannot participate in it. The bird cannot change its situation because it is literally imprisoned. The caged bird appeals in its pain to Heaven, hoping God will relive its suffering.
