I think we don't just have opposite personalities but we are a mosaic of several personalities that coexist and that are are reinforced by a number of factors such as experience, environment, social factors and especially by the presence of so-called influencers around us..it's true that we tend to follow what people around us do but I think that we tend to find "leaders" in our small communities and we are heavily influenced by them..it's how we are wired, we evolved in tribal communities and sometimes the survival of the whole community relied on selfish actions against members of other communities