How does C.S Lewis come to the conclusion that Jesus is God? 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 &quot;how-ever strange or terrifying or unlikely it may seem, I have to accept the view that He was and is God&quot; 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?
why do we come to say that he is God simply like that if he could also be a lunatic or something worse? 

Lewis, at this point in the book, is arguing against a specific view of Jesus - namely that he was a wise man, a good man, a great teacher - but not God! It's a widely held view among people who haven't looked closely at the Gospels, and it's a convenient one for people who want to acknowledge Jesus as an ideal, but either don't want to accept the supernatural or acknowledge Jesus as their Lord . So Lewis isn't looking to refute the &quot;lunatic&quot; or &quot;fiend&quot; viewpoints in that section. Neither is he looking to produce a &quot;mathematical&quot; proof.
So the point of the section of the book is that only three views of Jesus are reasonable. Which one you choose is left to other parts of the book, and the reader's own investigations.
As he says:

It seems to me obvious that he is neither a lunatic or a fiend.

He has read up about Jesus, both the stories of his life and what others have thought of him, and come to the conclusion that &quot;lunatic&quot; and &quot;devil&quot; are both off the table. Most people, even those who don't believe Jesus is God, would probably agree.