How come some people be mean to people close to them

I think people hold those who are close to them to a different standard than they do strangers. What's tolerable or even charming in a person you don't have to deal with on a daily basis can be terrible to live with or be around constantly. And I think a lot of people take the stress of their life out on the people closest to them (especially if they live together), and associate those people with that stress, and see strangers and people who aren't as close as an escape from their daily grind.