Complaints and controversies are most often caused by ignorance and misunderstandings. Lack of proper information often drives people to form a hostile opinion about another person. This is not the way to form an opinion about someone. Islamic teachings advise us to investigate the matter thoroughly to arrive at clear and convincing evidence, and based on that alone form an opinion.
In our social dealings, people complain about each other. These complaints get magnified, and soon turn into hatred. Hatred, in turn, escalates into uglier forms of evil, including enmity. In almost every such case, the fundamental cause is people forming opinions about each other without proper investigation and relying on rumours and hearsay, which they take to be confirmed facts. The results of this are always serious: regret in this world, and punishment in the Hereafter.