It's because we're poor, and she hasn't got any background.
"'Background ?'" Walter repeated.

"'Background ?' What kind of talk is that ?" "You WILL go with her to-night, Walter ?" his mother pleaded, not stopping to enlighten him.

"You don't understand how hard things are for her and how brave she is about them, or you COULDN'T be so selfish! It'd be more than I can bear to see her disappointed to-night! She went clear out to Belleview Park this afternoon, Walter, and spent hours and hours picking violets to wear.

You WILL----" Walter's heart was not iron, and the episode of the violets may have reached it.

"Oh, BLUB!" he said, and flung his soft hat violently at the wall.
His mother beamed with delight.