There are a few different themes found in D.H. Lawrence's Sons and Lovers: The Oedipus complex, bondage, contradictions and oppositions, and nature and flowers.
The Oedipus Complex- the name comes from the Greek tragedy of Oedipus Rex. It is prophesied that he will murder his father and have sex with his mother. Sickened by the thought, he leaves his home so that it may never come true. Through an unusual series of events, it actually ends up happening. Freud argued that the Oedipus complex is a real thing and is a repressed desire for most young boys. Sons and Lovers uses this complex to explore Paul's relationship with his mother, which sometimes borders on romantic. In addition, Paul hates his father with a passion and often fantasizes about his death. Paul tries to get rid of his guilty feelings by transferring his feelings to Miriam and Clara (transference also being a Freudian term). Paul is unable to love either woman as much as he loves his mother, which hurts his relationships.
Bondage- Lawrence discusses the idea of bondage or servitude in both social and romantic arenas. Mrs. Morel feels that she is bound by being a woman. This causes her to feel jealous of Miriam who is able to use her intellect. Romantic bondage is discussed in greater detail. Paul (and, to a lesser degree, William) feels bound to his mother and unable to love or marry someone else. He is obsessed with the idea of two people "belonging" to one another and he wants a woman to forcefully claim him as her own. No woman that Paul meets claims him as strongly as his mother does. Jealousy is discussed throughout the novel, which is closely connected to the theme of bondage.
Contradictions and Oppositions- The main contradiction seen in the novel is that of love and hate, as Paul goes back and forth between these two with all of the woman in his life. Sometimes he seems to feel both at the same time, as with Miriam. Lawrence also contrasts body and mind when exploring the idea of desire.
Nature and Flowers- Lawrence uses descriptions of the natural environment to explore the emotions and desires of the characters. There is often an erotic edge to the nature descriptions, which was a way to get his point across while still being modest enough to allow the book to be published in Lawrence's time. Lawrence's characters also experience moments of clarity and transcendence when they are alone in nature. Characters also tend to bond deeply while in nature. Flowers are used to symbolize these deep connections.