Different teachers provide exposure to various approaches. Moreover, each teacher exhibits different strengths and the best guide for practical halacha might not be a superior instructor for Tanach or Jewish philosophy. A rebbe great at analyzing a Talmudic topic could prove less adept at helping a student address a personal crisis. Therefore, a student must seek out numerous sources of education and guidance. Indeed, the gemara (Avoda Zara 19a) recommends studying analytical skills from several mentors even as it also calls for absorbing the basic material from a single teacher.
A number of commentators notice that Yehoshua calls for seeking out teachers and colleagues but says nothing about searching for students. Tosafot Yom Tov neutralizes the question arguing that the Men of the Great Assembly had already emphasized the need to raise students (Avot 1:1); therefore, Yehoshua felt no need to rehash the idea.
Others contend that the omission indicates a reluctance to seek out students. What could be negative about the desire to teach Torah? Maharal mentions concerns about potential arrogance. Teaching Torah reflects a highly noble pursuit, yet noble endeavors also allow for problematic expression. A teacher may be more motivated by the need to hear student adulation than by the desire to teach Torah or help others. Educators on an ego trip generate a number of negative outcomes. Such teachers may act manipulatively towards students, may care more about receiving praise than about aiding pupils, and may abandon personal and familial responsibilities in the quest for pedagogic fame.
Of course, none of the above is intended to denigrate the many wonderful and successful teachers of the Jewish community. I can think of few professions more important and I adopted this vocation myself. Nevertheless, the caution remains in place. Let us seek out multiple masters but avoid an unhealthy pursuit of disciples.