This is a requirement by the life insurance company which is called "Insurable Interest". It means you rely on the person you want to insure for some means of financial support, or you would possibly lose financially if the person were to die.
For example, if the person is your parent and you would pay for your parent's burial if he or she died, you have an insurable interest in your parent and can buy a life insurance policy on your parent.
Or, if you owe a business partner money as a loan on your business, your business partner may take out a life insurance policy on you.
However, when buying life insurance on another person you may or may not need that persons's permission, signature on the application for life insurance, and the person may have to answer some health questions for the life insurance.
Learn more about who can you insure for life insurance?