Why Vedanta requires to be learned from a teacher (guru) ?
There are multiple reference for the same in Upanishads, which has given importance to learn the self-knowledge from a teacher (Acharya)
In Kathopanishad, Yamraj explains to Nachiketa:
नैषा तर्केण मतिरापनेया प्रोक्तान्येनैव सुज्ञानाय प्रेष्ठ – Kathopanishad 1.2.9
हे प्रियतम नचिकेता! यह ज्ञान (मति) तर्क के द्वारा प्राप्त करने योग्य नहीं है, केवल अन्य के द्वारा तुम्हारे प्रति
कहे जाने से ही यह ज्ञान (सुज्ञान) प्राप्त होता है।
O Dear Nachiketa, this knowledge cannot be learned by reasoning, it can only be thought by someone to you.
In Chandogya Upanishad, Aruni explain to the Svetaketu :
आचार्यवान्पुरुषो वेद – Chandogya Upanishad 6.14.2
आचार्यवान् पुरुष ही सत्य को जानता है
Those who are instructed by a teacher, they only know the true knowledge.
In Chandogya Upanishad, Satyakama Jabala said to his Acharya :
श्रुत ह्येव मे भगवद्दृशेभ्य आचार्याद्धैव विद्या विदिता साधिष्ठं प्रापतीति – Chandogya Upanishad 4.9.3
“As I have heard from rishis that teaching gained from acharyas leads to the highest good.
In Bhagavata Gita, Lord Krishna says to Arjuna :
तद्विद्धि प्रणिपातेन परिप्रश्नेन सेवया |
उपदेक्ष्यन्ति ते ज्ञानं ज्ञानिनस्तत्त्वदर्शिन: – Bhagavata Gita 4.34
Learn the true knowledge by approaching a teacher. Inquire from him and render service unto him. Such an enlightened Saint can impart knowledge to you because he has seen the Truth.
