It's important to talk to your healthcare agent about the medical treatments you would want or those you would not want to have, how aggressively you want to be treated, and under what circumstances. You should also discuss how and where you want to spend your last days and what you want done with your body if you pass on. These final details are all important. But it's just as important that your healthcare agent know what you consider an acceptable quality of life. When your healthcare agent knows these very personal details about you, then the healthcare agent can make medical treatment choices that are in tune with your wishes.
And don't choose someone because you feel like you have to; instead, choose someone who really knows you. Then communicate openly and often.