I think it's an inherent part of human nature to express negative emotion; and some people take this to an extreme. What makes it more interesting to me is when this negativity is expressed with no regard for fact; at least not all fact(s). A lot of it comes from no real,valid source,and only serves to bolster that particular person's need to re-enforce their own self-worth.They feel the need to play their part in the perpetuation of the cycle of extreme negative emotion.
If they're given a point-by-point counterargument in response to their opinion; they will thoughtfully listen-or,at least appear to listen, but absolutely will not be swayed and maintain their negative stance now for no valid reason.
In other words,haters gonna hate.