Some novels are written with "messages", such as the anti-slavery message of Uncle Tom's Cabin or the anti-totalitarianism of 1984. Ulysses, though, is not a polemical novel with a "message." As most modernist novels, it develops out of the "art for art's sake" movement, opposed to the high moral seriousness of Victorianism, and is not intended to convey a single message. In fact, one could argue that it is novels without messages, because they are less tied to the specific issues of a place and period and communicate to a wider range of audiences than do novels with messages.
The most universal theme in the novel is probably that of the coming-of-age of a young man, Stephen Dedalus, and how his own maturation process exists in relationship to his feelings about his parents and the generation before him. Just as Stephen, himself a writer, grows up by reflecting on his own heritage, so Joyce, in setting his novel in relationship to Homer's poem, achieves artistic maturity in his relationship to his own literary tradition. The sections on Bloom and Molly also address the common problem of address what we might now describe as mid-life crises, and the impetus to both enjoy the security of established relationships and the desire for novelty.