Believe it or not, D.H. Lawrence rewrote this entire novel four times before he was happy with it. In most of the original drafts, he named the book Paul Morel. The shift to Sons and Lovers, though, makes the work less focused on Paul alone and more focused on all of the characters—on the messed-up dynamics of modern love.
Lawrence's drafts also successively shifted from being mostly about Paul to having more emphasis on characters like Walter Morel, Mrs. Morel, Miriam Leivers, and Clara Dawes.
The title also gets a lot of mileage out of that one conjunction, "and." You can read the title as meaning that a son can play the role of lover with another woman, for example. But Sons and Lovers might also imply something incestuous, in the sense that Paul is simultaneously Mrs. Morel's son and her lover.
On that note, you know who loved a good turn of phrase? Sigmund Freud. Maybe there's a reason for the popularity of the phrase "Freudian slip" after all…