Why doesn’t mama want Stacey to hear papa complaining about his leg

Mama is awaiting the return of Stacey, Papa, and Mr. Morrison from their trip to Vicksburg. When they finally arrive, Mr. Morrison is carrying Papa. Papa's head is bandaged, and his shotgun is strapped to his leg with a rope to keep it from moving. As Mama and Big Ma tend to Papa, Stacey shares the story of what happened.
Stacey tells the other children that they were on their way back from Vicksburg when the two back wheels of the wagon fell off. Since Papa feels that somebody is after them, they decide not to unhitch the wagon like they would normally do to place the wheels on. Stacey is asked to hold the horse's reins to keep him still while Mr. Morrison lifts the back of the wagon. As Papa places the wheel back on, he is shot. The horse becomes startled, and Stacey can't hold him steady. This causes the wagon to roll over Papa's leg.
The bullet only grazes the side of Papa's head, but his leg is broken from the wagon. When Papa shows his frustration with having a broken leg, Mama cautions him. She knows that Stacey feels responsible for Papa's broken leg because he couldn't hold the horse steady. She doesn't want Papa's frustration to cause Stacey to feel worse than he already does. Papa is angry that, although men tried to kill him, it is his son that feels responsible.