Why Does Plato Think That Philosophers Should be Kings?

The definition of a philosopher and the characteristics required of the philosopher-ruler are subjects of the dialogue Republic. Summarized it can be said that, according to Plato, human beings may reside in two worlds: the lower world of Belief and the higher world of Knowledge. While governance by non-philosophers would mean to be caught in the sensual world and therefore governed by mere opinions, beliefs and self-interest, the philosopher ruler will in contrast govern with virtue and justice without self-interest because of his/her special education in knowledge of absolute virtue, justice and other qualities.

As true philosophy means gaining the above qualities, philosophers are the only possible rulers. It is important to make a distinction between the acquisition of knowledge and the acquisition of truth, because knowledge is not necessarily the final truth. So philosophers of course can make mistakes, but will be ready (and hopefully able) to correct their views towards more truth.