Well, maybe party because of the Sociological Theory of Labeling Theory?
As it essentially works... at least I think it does.
Although in the case of labeling theory its saying [basically] that if someone does a crime, and you label them a criminal, they will always be seen as one - and they'll usually always be seen as a bad guy.
Similar to a sex offender.
You could make sexual joke in a work place, and if someone doesnt like it, they can take you to court. And lets say they win, you'll be labeled as a sex offender, even though what you did (in your eyes, and perhaps the eyes of many others) was fairly harmless. but because you have that label, people see you very differently.
So really, labels hold a lot of power. There's a big difference between saying (for example, Im going to use you iNow - I hope you dont mind ) "iNow is an environmentalist" and "iNow is a hippie tree-hugging liberal" - one is respectable, while the other is demeaning. Giving you a label can change how people perceive you, even though you've done nothing different.
The answer as to why we do it? I think its because it really does work - as long as enough people latch on to it and believe it.
If you hear someone is something (a right-wing nutcase for example), and you have no proof of it, its like a first impression, you're almost bound to believe it until proven otherwise.