Bipolar is one personality that flips between elated and depressed, or normal and depressed. Imagine your Missus when she has the decorators in and you just complemented her on her shoes or something. It's not the multiple persona thing that people do online talking to themselves, although a lot of that can be because they have no real friends, even online, so they create threads on rep forums where they discuss stuff with themselves to give the impression that someone cares.
Not that I'm accusing people here of being like that, obviously. That would be rude, and everyone here is perfectly sane and lovely. Obviously.