Lady Macbeth explains her husband's strange behavior is the result of an illness which has plagued him since youth:

Sit, worthy friends: my lord is often thus,
And hath been from his youth: pray you, keep seat;
The fit is momentary; upon a thought
He will again be well: if much you note him,
You shall offend him and extend his passion:
Feed, and regard him not. (3.4.64)