There are multiple explanations to this, and I'll try to add others as I locate them.
For now, see Rav Bartenura's who quotes Ramba"m. The term "rav" doesn't mean a "rabbi" necessarily, but a teacher. The point is that you have to appoint someone as a teacher so that you can learn from him. I believe that the idea is that you should learn not just halachot, Torah, Gemarrah, etc., but learn behavior as well. I.e., he should be a mentor.
He (Ramba"m) points a conflicting adage in Talmud Avodah Zarah 19 that states that one should not learn from just one rav, but from many. They resolve the conflict by stating that in Avodah Zarah, it refers to advice where it is good to hear multiple opinions. (E.g., if you needed advice on career choice, where to live, etc.) But, regarding learning Gemarah (I assume this applies to any Torah learning), it is best to learn from one rav so that you don't blemish your learning.
Avot D'Rabi Natan 8:2
היה רבי מאיר אומר: הלומד תורה מרב אחד למה הוא דומה? לאחד שהיה לו שדה
אחת, וזרע מקצתה חטים ומקצתה שעורים, מקצתה זיתים ומקצתה אילנות. ונמצא
האדם ההוא מלא טובה וברכה. ובזמן שלומד משניים שלושה, דומה למי שיש לו
שדות הרבה. אחת זרע חטין, ואחת זרע שעורים, ונטע אחת זיתים, ואחת אילנות.
ונמצא אדם ההוא מפוזר בין הארצות בלא טוב וברכה:
Paraphrased translation: Rav Me'ir said that one who learns from a single rav is like someone who has one field and he plants a bit of wheat, barley, olives and trees. That person is filled with good and blessings.
But if he learns from 2 or 3 people, it is like someone who owns many fields. In each field, he plants only one of each of the items mentioned above. In the end, he is a person scattered among all the lands with no goodness and no blessing.
The analogy, here, is that a single rav helps one organize his thoughts and gives him purpose to what he is doing. You can think of the planting of the seeds to being analogous to a person's thoughts, desires, and questions. They're all different. But directed to a single rav, that rav directs him how to organize all this and he gets a blessing by being focused and directed vs. getting multiple opinions and senses of direction from different people - even if they are all ravs.