In Macbeth, why aren't Macbeth and Lady Macbeth happy being king and queen?

Macbeth and Lady Macbeth are not happy being king and
queen because they do not feel that they are safe in their positions.  Macbeth remains
feeling ultimately guilty for killing first Duncan and then Banquo, and his guilt causes
him to have fits and hallucinations.  He tells his wife that he has been having
nightmares and that he does not have a general sense of peace.  The couple fear that
someone will find out what they have done, so they cannot rest easily.  Finally, Macbeth
fears that the second half of the witches' prophecy will come true and that Banquo's
line will usurp him to take over the throne.  How can the couple be happy when they are
in constant fear that their time in power is short-lived?