The Question; What is Self?
Self as a word has an old German version like selb, yet I would think self is uncomplicated etymologically and is something like cell- the cel(f) or sel(f) is just the mine(d) associated with the cell (like a brain, prison space or whatever. Self is a word describing (self) awareness location. The word pre-exists by centuries any kind of neurological science or even very accurate philosophical analysis concerning what it is made of. The Latin word for self is ‘sui’. Given a couple of millennia that’s reasonably close.
Bruno Snell wrote a good book called The Greek Origins of European Thought. He related some of the etymology of language one can discern in ancient Greek. He said that verbs describing actions and events commonly experienced evolved into nouns as an ossification. For example, the action of skidding something might become an eponym for a thing called a skid. I would guess that self evolved the same way as mine or mind and was evolved from practical use by non-philosophers.
On the Question; Why did Jesu reprove one who called him ‘Good Master’?
One might actually look at Mark 10:17-18 where the scripture is found.
17″And when he was gone forth into the way, there came one running, and kneeled to him, and asked him, Good Master, what shall I do that I may inherit eternal life?” 18″And Jesus said unto him, Why callest thou me good? there is none good but one, that is, God.”
The guy asking the question was speaking to Jesus as a genre of teachers- a “good master”. Elsewhere Jesus said to call no one teacher for one has one in heaven, and to call no man father, for God is the father. So Jesus has said in three different circumstances something of an instruction or correction on related topics. He pointed to God instead of mankind in all three instances.
What may have been a matter of timing directing the public to God, including himself, before the public actually knew he was God. The average soul of that day, and especially the Jews, had no idea about the triune nature of God. Like the wine commercial, Jesus would release no wine before its appropriate time; and there was a schedule leading to the cross to follow. Mark 8: “27 And Jesus went out, and his disciples, into the towns of Caesarea Philippi: and by the way he asked his disciples, saying unto them, Whom do men say that I am?
28 And they answered, John the Baptist; but some say, Elias; and others, One of the prophets.”