What evokes this particular response is the experience that Sylvia has had of wandering around an upscale toy store, an experience that brings home to her just how unfair and unequal society is.
The trip to the store was meant to be educational, and it was, though not as Sylvia's teacher Miss Moore intended it. What Sylvia learned from her trip to the store was that life is incredibly unfair if, like her, you're poor, female, and Black.
But the day out at F. A. O. Schwartz also taught Sylvia to be defiant, not to be beaten down by her lowly condition. That explains why, right at the very end of the story, she refuses to participate in a race with her friend Sugar.
After having been reminded all day of how society beats down people like herself, the last thing Sylvia wants is to be beaten in a race. She's had enough of losing for one day and wants to take time to think through her experiences of this unusual day.