In the saying, "When the student is ready, the teacher will appear," readiness means that the teacher appears to those who are ready to receive his guidance, not that he appears to those who already know the truth.
The key component of the process of internal spiritual preparation is having an extremely pure, earnest heart toward growth and awakening. Those who truly, earnestly seek the truth and want great understanding concerning life, often end up searching for a teacher who will help them.
A teacher for a spiritual path is one who has walked the same path before us, and who has obtained spiritual awakening by overcoming countless challenges on that path, and so can help us move toward self-realization. The teacher is, so to speak, a person who can play the role of navigator on a spiritual voyage.
A teacher is someone who, when we are struggling with ceaselessly changing desires and emotions, helps us see our shining true, changeless nature beyond the world of transient phenomena. He or she is someone who constantly reminds us that the moon is still shining, even when it is obscured by clouds in a dark, nighttime sky. A teacher can give you a push, enabling you to turn your outwardly directed gaze inward.
Is a teacher absolutely necessary for traveling down a spiritual path? In response to this question, I'd like to give you the following example. Let's say that you have had the dream of making beautiful watercolor paintings for a very long time. Naturally, you would probably be interested in information on watercolor painting. You would buy books, do research on the Internet, join a watercolor painting club, and go see exhibits by watercolor painters. If your passion for watercolor painting grew very deep, you would probably find a teacher who could inspire you and help you paint wonderful watercolors. The process of finding a teacher to guide you on a spiritual path is no different.
A teacher isn't necessary for everyone. What's important is what you want and how earnestly you want it. If you can get what you want for yourself, then you probably don't need a teacher. If you have a thirst for the growth of your soul, you will find a teacher naturally when you can't satisfy that thirst yourself.
There is a saying in Asia: "When the time comes for an egg to hatch, the chick pecks at the shell from the inside, and the mother hen from the outside." I'm not sure whether this really happens when a chick hatches from its egg, but this expression is often used as a metaphor by the spiritual traditions of Asia when describing the relationship between a teacher and a student.
Those who genuinely seek spiritual growth and completion and who earnestly wish to transcend momentary, sensual satisfaction to discover what is truly important in their lives, discover a being to guide them on their journey, a teacher. The teacher also recognizes a student who has such an earnest heart. Then, regardless of who goes first, if you and your teacher, from within and without, break away the hard shell of fixed ideas and ego, a new realization will appear in your life.