Notes From Reviewer

Title 

If the title is supposed to connect to the fact that life is very complex then good for you because it's not a false thing as I said down in the plot section. If the point of the description was to make it seem like it connected to the title, I don't think it fits.

Description & Foreword 

The description doesn't really make any sense to me. I tried to break it down and read it over and over again but I still don't get it.

When I broke it down by changing the words to certain things, it came out to: "It's unusual_{(extraordinary)} how one term can define into several_(numerous), delightful_(wondrous) difficulty_(complexity) that fills up our youth to its extent, Sana often thought."
I think the word complexity is making it not make sense. If you omit it and change it around: "It's extraodinary how one term can define into several different terms that fill up our youth to it's extent, Sana often thought."

I honestly can't tell what you were aiming for within the description so I tried to make it make sense while keeping what you had. Using the word complexity in the description really wasn't necessary. Complexity means difficulty. It just didn't fit. If you want, you can try to explain it to me when you pick the review up but I really don't know.

Plot

I like the concept of "complexity" because really, life is pretty complex sometimes especially when you are gay/lesbian/trans, etc. life can be pretty difficult because people are scared of coming out, not being accepted by family, etc etc. (Which I think by the A/N that Sana's family is against(?) the LGBT issues or just don't want it in their family (I'm really not sure) which can put pressure on Sana for liking Tzuyu which is why I guess she smacked her? I don't know - I'm trying my best to understand what happened here.

Writing Style 

You have a sentence that contradicts itself in the oneshot right above the word "Chou Tzuyu"
You said: "...the biggest question that remained to be unsolved is about life and love in general."
It should be: "...the biggest question that remained to be solved is about life and love in general." 
The "question" is unsolved, so therefore the question needs to be solved, not unsolved.

Instead of saying "*smack*" or something, you probably should have explained that Sana had slapped her. It's better to describe things since that looks like text someone would use during a roleplay. With the text in color, I didn't really find that necessary but I can see why you did it.

Characterization 

When it comes to oneshots, it's pretty hard to get a firm grasp on characters but from what I have seen and been told, Sana does suffer from insomnia and Tzuyu is a caring person who wants to help Sana. It seems like Sana has a lot on her mind which causes her to slap Tzuyu on impulse even if she really didn't want to deep down.

Flow 

Not too shabby, I guess. You got the point across which counts for something. 

Personal Enjoyment 

I don't like Twice so it wasn't my cup of tea and I usually don't read yuri fanfics since I only ship limited yuri couples like jensoo (jennie/jisoo) and rosa(??) (rose/lisa) so of course this wouldn't be something I'd normally read. I got the concept you wanted to get across to the readers but yeah, not my type of story. Other people seemed to enjoy it though so that's good.