This is in order to prevent life insurance from being used for nefarious purposes, such as taking out a life insurance policy on a stranger without his or her knowledge and then hoping to cash in on it, or worse yet, cause that person's death.
Insurable interest regarding life insurance means the person for whom you are purchasing the life insurance must be a spouse or other close relative that you rely on for some form of financial support, or whose death would cause you some form of financial loss.
For the purposes of purchasing life insurance, all persons are assumed to have insurable interest in their own lives. Which means you can buy life insurance on yourself.
Generally speaking, even if a person has an insurable interest to you, such as a spouse, or business partner, you cannot take out a life insurance policy on him or her without his or her knowledge and permission.
Learn more about who can you insure for life insurance?