where his civilizations, including our own

I can see Harry having the idea that crying openly isn't a good thing reinforced by living with the Dusleys. Though years of living in that kind of environment could have made him resigned to his "fate", Harry is instead defiant with the Dursleys - and becomes more openly so with each book. He may have learnt very early to hide any effect the Dursleys' treatment had on him. Perhaps his way of fighting back was to not let them see that their unkind words bothered him in any way.