The last sentence kind of explains the use. When it is used without an object, it generally means to lack something desireable or essential. Consider the following: He takes good care of his children. They want for nothing." It doesn't mean that there isn't anything his children wants (they may want expensive toys for Christmas). It just meens that they have everything they need. and My girlfriend and I have a wonderful relationship. I want for nothing." The sentence you ask about: Who can be considered healthy who wants? can also be written as follows ( [] shows implied meaning): Who that wants [for something], can be considered healthy? Does this help?

Another way, a more 'normal' way to write the sentence is: Can he who wants be considered healthy? "who wants" is a relative clause. The passage is from the book *Delirium* by Lauren Oliver. It is about a society in which love is seen as a disease. Since love and desire (wanting) cannot exist apart from each other (per the passage), whoever wants or has desire is also seen to have a disease.