Grammatical polarity is noted by the negative. Jane is not here. Her absence is marked by not but her presence only by the absence of not. When Jane is here, no word is added to indicate as much. Positive polarity is thus notable only for its lack of negativity.
When Jane thinks about this, it saddens her, seems indicative of some profound problem people have, deeper than grammar (if that is possible), something skewed about humans, so Jane resolves to be positive, hoping more yes will bring out the best in her. As in, Jane is yes here.