Split/Dual Personality..

Split or Dual Personality....
All of us might have come across this term quite often.
And as soon as we say use it with respect to someone, the first impression that we USUALLY get is that the person is suffering from some disease.
But a deeper and closer look will make it more clear that all of us suffer from this disease most of the time. Its just that most of these times our actions are among the norms of the society and thus go overlooked.
At times we do things with a feeling of 'Just Do It...' and then analyse it later, if what we did was right or wrong.
The same person at some other point in time will first analyse and then act accordingly, for a similar situation.
Why is this difference in approach. I ask myself the same quation so many times and every time i come across with different reasons.

Can any of you help me with some reasoning for this?