The doctor feels like Lady Macbeth needs help from god more than anything he can offer. The phrase "Unnatural deeds Do breed unnatural troubles" shows his stance on what afflicts Lady Macbeth and the cause of it, and how he things he's unable to help her with any common medicine. Lady Macbeth's condicion, brought upon her by the guilt and grief caused by her hand in the assassination that takes place earlier in the play, is more psychological than anything else, and it makes sense that Macbeth's physician would think only god could cure something like that.
