How did Henry’s mother behave when he left for war? Why did her reaction disappoint him?

His mother had discouraged him from joining but he enlists anyway.

When Henry finally leaves, his mother does not try to convince him to be a hero, as he expected. Instead of an impassioned, beautiful scene, his mother gives him some simple advice. She tells him to be careful and not try to beat the entire rebel army himself and not to fall in with a bad group of soldiers. Then she adds: "I don't know what else to tell yeh, Henry, excepting that yeh must never do no shirking, child, on my account. If so be a time comes when yeh have to be kilt or do a mean thing, why, Henry, don't think of anything cept what's right." Henry is a little disappointed, he expected a more heroic goodbye.