quick - no one's looking!

Inspired by an office colleague who earlier today sent around an e-mail declaring that he needed to scratch his groin area but couldn't because there were people around, I'm wondering why it is that we develop habits that we know are embarrassing? For example - at an early enough age, most of us realise that picking our noses in public is not good manners. Yet an awful lot of people still do this in private. You would think that the habit of not picking your nose in public would start to rub off on your private time. Apparently it doesn't.

Before I get a torrent of comments (i.e. two) about this please note that I do realise that some things just need to be done and it's not a case of whether its good form or not - an example of such a situation would be how men have to stare intently at a scantily clad, good looking woman. It just needs to be done.