It is impossible to receive spirituality (meaning, to acquire the quality of bestowal that’s similar to the Creator) just by studying the books by yourself. You need a live teacher because only he can connect your present level with the spiritual level. The teacher has to serve as the intermediate level between you and the Creator. On your own, however, you will never be able to “jump” up to the Creator’s level. Your teacher is the intermediate degree between you and the spiritual world. He is able to do this because he exists in both worlds simultaneously.
The Ari and Baal HaSulam both write that intermediate states are the means to shift upwards. For example, see Part 3 of Talmud Eser Sefirot, “Inner Reflection.”
Even if a student could ascend to the first spiritual level on his own, it would still take him a very long time to learn about the Upper World without a teacher’s help, and then he would not have enough time to correct himself. Just like in this world, first you have to grow up for about 20 years, and only then can you begin to live on your own. But if you did not spend the first “20 years” of your “life” next to a teacher, then you are like a child who never received an upbringing or education, and who grew up without any support from adults.
That is why a person must listen to what the teacher tells him and make every possible effort to carry out the teacher’s advice. The student should accept this advice above his own egoistic surmises. Don’t be like a child who refuses to open his mouth and accept food, since what will the teacher be able to give you otherwise?