In the final chapter, the narrator suggests that love and hate are, in many ways, the same.  He says, 

It is a curious subject of observation and inquiry, whether hatred and love be not the same thing at bottom. Each, in its utmost development, supposes a high degree of intimacy and heart-knowledge; each renders one individual dependent for the food of his affections and spiritual fife upon another: each leaves the passionate lover, or the no less passionate hater, forlorn and desolate by the withdrawal of his subject. Philosophically considered, therefore, the two passions seem essentially the same, except that one happens to be seen in a celestial radiance, and the other in a dusky and lurid glow. 

Unlike many novels, Hawthorne tells us main theme or moral in the last chapters. Hawthorne writes, "Among many morals which press upon us from the poor minister's miserable experience, we put only this into a sentence:—“Be true! Be true! Be true! Show freely to the world, if not your worst, yet some trait whereby the worst may be inferred!” In other words, do not be a hypocrite. Do not put on a false front to the world to make it seem like you have no faults. It is OK to let others know you are not perfect. This idea is exemplified in the life of Dimmesdale and Hester. Dimmesdale tried to hide his sin and guilt from the world. As a result, he was eaten alive by remorse and guilt, his heart literally weakened, and he died. Hester, on the other hand, never hid her sin and learned to rise above it by becoming humble and doing good deeds. She becomes stronger and more respected in the end that Dimmesdale, who loses all credibility with the people of Boston at the end of the novel.