They Taped Over Your Mouth

I will never understand why people are the way they are.  There are these habits that seem to be engrained in people's heads and despite their best efforts to try and get rid of them or improve themselves, they can't.  Maybe it has to do with what my co-worker was talking about, working on strengths and trying to develop them.  I know that a character that is completely without weakness is a terrible idea (I know Mary-Sues were always bad in the role-playing game world - ick.) but sometimes it isn't easy to create a character that is believable and not a walking cliché (Thank you Professor Westermann).

But people have these quirks and ticks that make them "normal" by societies standards.  And then there are the ones that are weird or strange, something that seems to be a completely relative term to anyone who is looking at a person.  So, how as a writer, do you approach this?  Does it have to do with how you personally view something or do you go with the social norms?  It's the kind of thing that seems to make my brain hurt.  A lot.