. Why does Mrs. Dubose suffer from “fits” when Jem and Scout visit her?

She was addicted to morphine.

Although she had fits, her fits were in result of her morphine addiction. She knew that the morphine addiction was not the right thing to do for herself, and wanted to end her life not having anything as a responsibility, so she stopped taking morphine even though it meant she would die. She is very courageous in doing this because it means that she dies having nothing against her, and that she does something anyway even though she knows she is licked, or almost certain to lose.