Why do people behave weird??

Over a last couple of months I have seen a few people behaving weird.well am not talking about any national issue of importance but am just talking about a few friends of mine.They are like typical politicians(no offence meant guys) who become the epitome of goodness one day and the following week they do all the things they claimed were not good.And I dont wanna restrict the topic of discussion to my friends alone.a lot of us do it over a varied period of time.Why do we do it...why does our brain tend to forget the very things that it claimed were bad at another point of time and make us do it;I am looking around me for answers.hope I do get one.