Eros and romance
The undeniable flavor of the novel is sexual and erotic. When Golaud meets the beautiful maiden, she is bent over, playing helpless, which makes Golaud feel helpful and powerful. This is the basic function of erotic attraction—to bind lovers by offering each other what they want emotionally and physically. In the novel, Mélisande represents the power of erotic instinct, and also it's decay into boredom, and the temptation of affairs, just because falling in love is more fun and interesting than the daily chore of marriage. Ultimately, her failure to master her erotic aspect leads her to fall prey to her temptations, and her death, although not a fair recompense for her actions, is ultimately the consequence of her own actions.
Vengeance and justice
The novel pairs the forces of love and sexual attraction with the powers of vengeance, loyalty, and justice. That doesn't mean that the story necessarily condones one above the other, but instead, the plot shows a cycle of behavior that is ultimately unjust. By not controlling her erotic compulsions, Mélisande invokes her own fate—not to say that murder is an acceptable response, but the murder symbolizes something important—actions have consequences, and often getting what you want now also means sacrificing your long term interests.
People usually don't get away with bad behavior
The novel presents a careful, cautious couple who is still discovered, as if to say, nobody gets away with doing the wrong thing for very long, because the consequences of those actions are usually enough to trigger suspicion. For instance, if Mélisande sees her lover, she is likely to act happier and more confidently, in a way that causes Golaud to suspect that maybe she's meeting her needs in other ways, perhaps meeting her romantic needs with someone else. There's just no hiding it when you're in love.
Faithfulness versus unfaithfulness
The story highlights a moral dilemma for Mélisande. Should she follow her heart and the passions of romance? Or does her moral obligation to her marriage precede her own happiness? This question isn't solved directly, but when Mélisande dies, there is the moral implication that faithfulness to her marriage would have spared her and Pelléas of their fates.
Royalty
This play belongs to a European tradition of making plays with important, often royal characters, perhaps for dramatic effect. Everyone in this story is regal and before she is even in the royal family, Mélisande owns a crown, which shows that her family is powerful and wealthy.
