Actually, that is something I noticed: Many people are scared off for good if they don't understand something at once. Most of my physicist peers don't understand things at once, it's normal. The question is how to deal with that - do you run away or do you try again, and what are your tools for that?
I'm not sure what triggers the need to argue in people. It's just so far removed from the ideal most of us strive towards in discussions, which is trying to understand things by all means possible, and not defend a preconceived notion. This sounds obvious and easy, but for many people, I found it's not. They identify so much with their state of knowledge and arguments (not to say opinions, which I can kind of get) that one has to tread very carefully. One wrong expression and they turn into fighting mode, and the discussion turns from a search for understanding and rightness into a match for which derailing is a valid technique.
Lack of focus is also an issue, and can be a derailing method. (Ask questions faster than they can be answered and don't listen to the person trying to explain. In the end console yourself that we can't really know anything and thus all questions are unanswerable. End monologue, walk away. God, these people.)
As far as physics topics go, it is the worst in GRT (general relativity) lectures followed by quantum things. Biologists told me they get strange argumentative older people in evolution lectures too.
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