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Sarvaguhyatamam bhooyah shrinu me paramam vachah; Ishto’si me dridhamiti tato vakshyaami te hitam.

Hear thou again My supreme word, most secret of all; because thou art dearly beloved of Me, I will tell thee what is good.
This is the beginning of the last 15 shlokas of Bhagavad Gita where Lord is planning to tell the most important secret that a person should know and something that a person needs to carry as the rendering of Gita is getting complete. He tells Arjuna to listen to his Supreme instruction very carefully as it is the most secret of all the things that Arjuna heard so far. Then Lord goes on to talk about why He is revealing such knowledge to Arjuna. Lord says that as Arjuna is beloved and closest to Him, that is why He is giving that knowledge to Arjuna.
Extending Interpretation to Mankind’s Life!
Not everything needs to be known to everyone. People who are knowledgeable and wise decide what kind of information to be shared with whom. A very apt example to take here is what a teacher reveals to his students. Though teacher imparts a lot of knowledge to all his students, there is always a particular student or a set of students who are closest to the teacher. This closeness is not dependent on any other factor, but the ability of the student to comprehend the knowledge that is given by the teacher. Not every student is capable of doing that and only those who are capable of it, a teacher gives the knowledge to that student. 
If you notice in this scenario, all the pandavas might be facing the same dilemma like Arjuna on fighting their own gurus, their own family members and those who were close to them. But it was only Arjuna who was able to express to Lord Krishna and from then on Lord starts to advise Arjuna which is something we all heard for 18 chapters. Now, Lord is convinced that Arjuna is capable of hearing the last and final sermon that He wanted to give. This confidence comes from the fact that Arjuna was able to so far understand everything that He told and hence he will be able to comprehend what Lord is going to tell now. We now await what Lord Krishna says to Arjuna.