When it happens
By the early severe stage
Why it happens
The ability to make comparisons and judgments, and retain information, has been lost. Even habitual decision-making (such as choosing when to eat or relieve one's self) disappears.
What you can do
Become an ace anticipator. Anticipate what your loved one wants and needs as best you can and make the decisions for him or her.
Don't feel you have to make a ruse of getting input once he or she has clearly lost the ability to decide. Make things easier on both of you by just choosing the food, clothing, or activity yourself, and presenting it in as respectful and upbeat a way as you can.
Don't be upset if your choices are rejected. It may be that your loved one has a different preference -- and that's great. Offer something else.
Rely on your best judgment of the types of things he or she once clearly liked. If you're not sure, just make an educated guess.
Remember that familiarity and routine can be comforting. There's nothing wrong with serving repetitive foods or doing the same activities multiple times, as long as they don't seem to invite boredom. (Be sure to keep the diet varied enough for good nutrition's sake, of course.)
Whatever you do, avoid making it "quiz time" once someone is in the severe stage of dementia and simply *can't * decide. Don't try to walk the person through choices, explain them or rationalize, or wait an extraordinary amount of time for a decision.