The psychological dimensions of Lady Macbeth are intense and intricate. The reader can identify many reasons why Lady Macbeth becomes insane over the course of the drama. One of the most direct would be due to guilt. She feels remorse for the murders that have taken place. She begins to recognize her own role in this process and how this process, itself, was wrong. This becomes evident as early as Act II:
Alack, I am afraid they have awaked
And ‘tis not done. The attempt and not the deed
Confounds us. Hark! I laid their daggers ready.
He could not miss ‘em. Had he not resembled
My father as he slept, I had done’t.
She has changed from the woman who claimed to be able to kill a baby at her own breast to a person who begins to display empathy. Lady Macbeth begins feeling guilt and regret. As a result, she is no longer able to rationalize what she and her husband have done. She is no longer able to absolve herself of murder. Such emotional realities play a role in her insanity and eventual death.
I would also argue that Lady Macbeth's estrangement from her husband is another reason why she goes insane. The closest we see husband and wife is in the first act. By Act V, both of them are living separate lives. His is one consumed with power and consolidating his control over the throne. Hers is filled with remorse, pain, apparitions, and obsession with what has happened in the past. In Act V, scene 3, the Doctor suggests as much to Macbeth: "Therein the patient Must minister to himself." When the Doctor suggests that Lady Macbeth needs spiritual counsel, it is a form of condemnation of Macbeth's absence. The Doctor's diagnosis suggests that Lady Macbeth needs someone to be there for her. Lady Macbeth needs an emotional resource because her mind and soul are "infected." Her lack of support from her husband can be seen as accelerating her descent into madness.
There are several possible reasons that this happened. It could have been over grief, remors, guilt, and there could be other interpretations over why this happened.
Lady Macbeth eventually becomes insane as a result of guilt, and the stress of trying to get away with murder when Macbeth is going crazy as well and making it harder and harder for her to hide their unspeakable deed.