Teachers are inherently insightful because they seem to be connected to and in harmony with the source of all creation. They are intuitive in ways that they can't explain themselves. They also effortlessly connect with other people and empathize with them - feel what they are feeling - naturally. That's why, people feel drawn to a teacher in a very personal and intimate way (unlike a mass of people being drawn to a charismatic personality.)
Teachers live among us and yet are somehow able to not get caught up with life and living in ways that most of us do. They are naturally able to keep a deeper, broader, more meaningful perspective on things, people, circumstances and events.
Guinan has all of the personal characteristics described above. She lives on the ship, works with the crew, mingles with the staff and yet somehow remains above it all. She is detached yet empathetic, involved yet aloof, conversant yet alone, drawing on her own internal resources to keep her rooted and nourished. She is present, observant, and always, always emotionally available.
Teachers are not without personal stories. (Guinan is one of the few survivors of an annihilated species that used to have billions of members.) Yet, teachers somehow have the ability to transcend their human experience and gain insight about the world from an objective perspective, the perspective of an observer.
Teachers don't always work as teachers, although many do. Nor do they always have an official position of a teacher. Guinan is a bartender on the ship. Yet no one commands more respect from the Captain of the ship than her. She is the only person on the ship who can walk into the captain's quarters and tell him that the decision he is about to make is wrong.