Bhagavad Gita Verse 64, Chapter 18

sarvaguhyatam bhooyaha shrunu me paramam vachaha |
ishtosi me dridhamiti tato vakshyaami te hitam || 64 ||
 
Listen from me again, my supreme statement, most secret. You are my strong friend, that is why I will speak what is beneficial for you.
 
sarvaguhyatam : most secret
bhooyaha : again
shrunu : listen
me : from me
paramam : supreme
vachaha : statement
ishtaha : friend
asi : are
me : my
dridham : strong
iti : in this manner
tataha : that is why
vakshyaami : I will speak
te : you
hitam : beneficial
 
Every teacher has their own style, their own philosophy, for teaching complicated subjects. A good teacher is one who can not only impart the knowledge, but also, extract the big ideas out of the complexity and present it to the student in such a manner that it can be easily grasped. There is no point teaching a complex subject which students memorize for the sake of passing the exam, but forget it as soon as they submit their paper. Shri Krishna wanted to convey the essence of the Gita to Arjuna, so he addressed him again, after having concluded the teaching.
 
The Gita is a formidable text to understand thoroughly. It deals with karma yoga, bhakti yoga, jnyaana yoga and raaja yoga. The same word is used differently in different contexts. For example, the word "aatmaa" is used to mean body, mind, intellect and the self. The Gita makes references to individuals from Vedic and Puraanic literature which requires further effort on the part of the teacher and the student. Seekers who approach the end of the teaching will appreciate a teacher who will extract its essence.
 
Shri Krishna also expresses his motivation for summarizing the teaching. He does not want any reward from Arjuna in return for this teaching. Nor is he doing so out of fear of anyone or anything. He only has a lot of affection for Arjuna, he regards Arjuna as a staunch friend, and he also knows what is the right teaching for the situation Arjuna finds himself in. Therefore, he wants to impart the supreme statement, the param vachaha, the essence of the Gita, that will provide the most benefit to Arjuna.