Woe be to him that reads but one book
To have a balanced understanding of a topic it is usually necessary to consider other peoples' opinions, even though you might disagree with them.
For example the present debate on global warming strongly divides opinion. Some people are totally convinced of dire danger and other, equally intelligent, regard it as scare-mongering exaggeration. Those who have only read and absorbed one side of the argument will tend to be intolerant of any other view.
Most situations in life are a compromise between extremes but you need to be familiar with both sides of the debate to form a balanced judgment.