Well, its definetely the normalisation of this abuse makes people think that being toxic is a part of normal life and is just somebody caring about you, when in reality it isn't healthy at all. Children base their attitudes a lot around those around them and you can see this clearly when somebody points out how somebody sounds like one of their parents, they are subconciously doing the same thing that there parents are doing since they have been imprinted on as children. I know that my awful jokes are representive of the fact that my father makes the same awful jokes (and also my anger issues that rarely pop up, though I have tempered it down a lot since I realise that is no way to treat people and acting like my father does only pushes people away) and others also have similar things. When adults are being toxic, they believe that it is normal behaviour and thus become what their parents are.
There are other things that can shape a person into a bad person, such as personal tradegies/experiences and other close relationships (mainly friends or role models) acting as bad influences, so it isn't a black and white scenario, lots of things can influence somebody to become a bad person and even if there are all the above things, a person can turn into a wonderful human being.
This of course, does not excuse anybody from acting this toxic way. As I like to say, "That's a reason, not an excuse, to be an awful person".