I have a feeling that most of the people reading this site already understand everything in this article, but it's a useful synopsis of common issues faced when trying to have a reasonable discussion with laypeople, and might be good to point them to if necessary.
http://thoughtcatalog.com/2011/how-to-have-a-rational-discussion/
I also want to mention how much I wish someone had shown me something like this as a teenager- I was very prone to lecture others against their will- as it might have saved me a lot of grief. I'm curious to see if these tendencies might have been common among members of this community growing up, so please comment to tell me if so (actually, please tell me even if not-no reason to encourage my own confirmation bias)!
IME, in these situations the answer is typically "Because I don't want to accept the status-hit associated with being perceived as not caring about the evidence." That is, I would rather think of myself as someone who evaluated the evidence and came to my conclusion, rather than someone who came to my conclusion without evidence.
Which makes it tricky to get people to admit to it when doing the latter, since that's just yet another way to take the status hit.
Lowering the wall can help a little... that is, artificially reducing the perceived status hit of admitting to not caring about the evidence.