Imagine you wake up one morning to find that a team of doctors are surrounding your bed -- and that they had attached a person to you through a series of tubes and other medical mechanisms, who was now laying next to you in your bed.
They explain to you that this person had to be hooked up to you, and that the person must be hooked up to you for 9 months, or s/he will die.
Your say in this matter is immaterial; the medically-needy person's rights override your own.
A bit miffed, you say it isn't fair that you must be forced to have this dependent person attached to you for nine months; it seems to violate a notion of self-determination, your sovereign control over your own body/being/self. "Sorry, it is illegal for you to kill someone," the doctors sting back, and shrug their shoulders that, well, it sucks to be you, but you'll just have to put up and shut up, and have the person attached to you for 9 months, regardless of your feelings about it.
-B.