Many personality traits can be observed but there are other hidden forces that shape who we are.

We usually think of personality in terms of the differences we can see – Sarah is smiley and chatty, John frets a lot, while Noah is neat and tidy. These differences are fascinating, but if we focus only on observable behaviour, it doesn’t tell us much about the roots of personality.
Looking inside our body gives greater clues. And a trickle of recent findings has recently turned into a torrent, as studies are now revealing how personality is linked with many aspects of our biology, from our hormones and our immune system to the microbes in our gut. 

Source: Yahoo News