Your description can be described in one word; 'bland'. While it has everything in the story summarised into a few sentences, which was good, there wasn't much feeling in the sentences. It's like you were vomiting the words out, which reassembled into sentences. 

While the last sentence did arouse my curiousity, the other sentences could be arranged into a paragraph each, which would make them more visually pleasing to the eye, because right now it seemed like a mess. 

You can hit the precise points of your story's , and put it in the description to make the reader have the desire to click into the next chapter. If I were a normal reader stumbling upon this fic, I wouldn't have much motive to click onto the next chapter. 

How about this; Wonshik is an aspiring artist who was drowning in the pools of despair at his latest work, which he never seems to be satisfied with. Until he realises how thin his bathroom walls really were, and found the other broken piece to his own. 

It sounds better, doesn't it? Your original description gave me the wrong idea at first; it mentioned how thin Wonshik's bathroom walls were, so I thought it was a fic (I apologise for my dirty mind) 

You don't have to follow my example, but try spicing it up a bit. I'm sure you can do it, seeing that your writing isn't bad at all. 

Everything looks fine to me, besides the jumbled mess in your description. You can change the font to a more elegant one, like Georgia or Lucida Grande, but it's fine as it is right now. 

The plot is fine at first glance; there weren't much plot twists, and the events were pretty realistic. However, when I read it again, I found a few points that I'm not sure about. 

For one, a piano bench isn't that long; with two people sitting on it, especially two people with Taekwoon and Wonshik's sizes, their bodies would've easily touched. I wish you could've gone into more detail in that. 

Another would be the wedding invitation. I know this fic is based off Beautiful Liar to some extent, but just from the story, I'm curious to why his ex would invite Taekwoon to her wedding still, even though they broke up. Is there a specific reason as to why? I wish you had put that into the story, just a comment here or there, not much but letting the reader know as well. 

Also, how did Wonshik know his way around in Taekwoon's apartment? Assuming he has never been there, even if Taekwoon's place is small, his feet wouldn't automatically move in the direction of the other's kitchen. 

The theme of your plot is certainly overused; neighbours who fall in love with each other after a single or few encounters are seen everywhere. However, I didn't expect the broken part at all. It was your twist to a cliche plot, and I really liked that. 
 

The details were written shockingly well, especially since your description didn't really gave a good impression at first. The shower scene and Taekwoon's feelings, everything was described in great detail, which I really like. Good job on that.

The flow is at a comfortable pace, and the story is easy to follow. There are no changes needed to be made, but your words seemed jumbled, as if everything's in a rush. However, the development of the story is at a comfortable pace.

Fellow starlight here~ So honestly, I'm not really a big fan of Wontaek. But this story is easy to read, especially when I can picture that sort of happening in real life? Wonshik is a dork after all XD. This story is a nice one to snuggle up in a blanket for, (aka stories which I can read when I'm de-stressing myself) so I really like it.