Across situations, when someone is, for instance, bugged by something and doesn’t behave as he usually does, we say he ‘is not himself’. When I’m stressed about an upcoming presentation at a party, my friends will say I ‘wasn’t myself’.
Maarten van Doorn
Because we already associate him with a pattern so he or she is predictable to us and we know how to interact with them. In other words, we have control over them in some sense and we can reduce the probability of them inflicting pain. When someone does not behave as he is supposed to do on daily basis we have to recalibrate our perception of him, thus we have to put in an extra effort which is bothering if not annoying. Most of the time unpredictable means risky and by nature we avoid risks.