Why does Kumalo get angry at his wife when they are discussing the letter?

The letter's contents reveal that Kumalo will need to make an emergency trip to Johannesburg. The only money they have has been saved for their son's education, but their son is longer with them as he never returned from a trip he'd made there himself.

Kumalo's wife tells him there's no longer a reason to keep the money aside as Absalom will never return to them. He becomes angry at her negativity and lack of faith, but then realizes that he's hurting her with his anger.