I am afraid media presenters often fail to appreciate this when they talk about personality issues. In fact they even tend to confuse DID or Dissociative Identity Disorder with Schizophrenia which is another even more different disorder and one which is usually NOT characterised by splitting of personalities but rather by sudden mood swings and other quite different processes. Let's try to clear this up.
Dissociation is a powerful process, but by itself it can be normal.
Day dreaming during a boring lecture is a typical example. We leave where we are and take ourselves and our mind somewhere else more interesting or exciting. The less we have to be aware of it the easier it is to deny that it is still a problem, but somewhere in the backs of our minds we will still worry about it. These forms of dissociation are however still within the 'normal' range of human behaviour, though that is not to say that they can't still cause you lots of problems. Children learn to dissociate as a normal function of playing and also to help them deal with childhood stress.