Final answer:
Plato left politics due to concerns about the messiness of democratic politics, promoting the rule of those best suited to govern. Education of future leaders and the concept of philosopher-kings were important to Plato's political philosophy.
Step-by-step explanation:
Plato left the political sphere because he was concerned about the messiness of democratic politics. He believed that democracy promoted and rested upon the cult of the average, where the majority of people determined everything by voting, without the impulse towards self-criticism and self-improvement. Plato preferred the rule of those who were best suited to rule, rather than having people ruled by kings, commanders, or tyrants. He believed in the concept of philosopher-kings, who combined practical knowledge with a deep understanding of intellectual concepts, and that education of future leaders was of paramount importance.