NO REASON TO CLOSE THIS QUESTION
The reason why this question is not opinion based, is because I am asking how C.S Lewis himself arrived to the conclusion of his dilemma. Because as it reads from this excerpt, his conclusion is arbitrary. Now, that's not to say that his decision truly is arbitrary, maybe there is something to be said about his decision from citations from his other works. But the question remains the same, I am asking it to be point outed how C.S Lewis himself came to this conclusion, there is no matter of opinion involved.
Now to the question itself:
Reading Mere Christianity by C.S Lewis, Lewis tries to go point by point from merely observing reality and our nature to arriving at the Christian God, so cutting to the chase -- he basically says that by observing ourselves we come to conclude that we are under an inescapable Moral Law of good and evil. He eventually goes on to say that this is evidence of a higher power, and eventually God, and evidently that this is a God that takes sides and espouses good, and so he narrows it down to Judaism, Christianity and Islam which are centered around a God that espouses moral laws of good and evil. This is where I start to really disagree with him, he narrows it down to say that it is Christianity to be chosen but he doesn't address why Judaism and Islam fitting his standards were unfitting candidates.
In any case, he goes on to say the following:
I am trying here to prevent anyone saying the really fool-ish thing that people often say about Him: ‘I’m ready to accept Jesus as a great moral teacher, but I don’t accept His claim to be God.’ That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic—on a level with the man who says he is a poached egg—or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God: or else a madman or something worse. You can shut Him up for a fool, you can spit at Him and kill Him as a demon; or you can fall at His feet and call Him Lord and God. But let us not come with any patronising nonsense about His being a great human teacher. He has not left that open to us. He did not intend to.
We are faced, then, with a frightening alternative. This man we are talking about either was (and is) just what He said or else a lunatic, or something worse. Now it seems to me obvious that He was neither a lunatic nor a fiend: and consequently, how-ever strange or terrifying or unlikely it may seem, I have to accept the view that He was and is God. God has landed on this enemy-occupied world in human form.
C.S Lewis then goes on to his next point explaining the basics of Christian doctrine and theology, treating the matter established -- yet he has not. He gives us a dilemma, that Jesus is either a lunatic, truly God, or something worse, then says "how-ever strange or terrifying or unlikely it may seem, I have to accept the view that He was and is God" then treats it as a closed cased, but hold on, why do we come to say that he is God simply like that if he could also be a lunatic or something worse? He made this huge leap in his argument without giving any valid reasons, yet he simultaneously acknowledges that it is strange, terrifying, and unlikely.
So why does C.S Lewis conclude that Jesus is God?