That is not always the case. There are some who only focus on the negative (or only on the positive) and do not always back up their opinions with sound reasoning. Also, when all one writes about is grievances and complaints (negative), then I tend not to take their opinions as seriously. It is similar to the idea of "Crying Wolf". Eventually, I tune out the negativity from those who are always negative. The same is true for those who are always positive.
When someone is neither all positive or all negative, I am more inclined to take their viewpoints seriously.