There was this song by f(x) featuring Do Kyungsoo who also has the same title as this film, Goodbye Summer and there was this line on the song that says:
The feelings I’ve hid still remain as a painful secret memory
The photos that can’t define our relationship is a heartbreaking story
I’m sorry, summer, now goodbye
This line somehow interprets the film's plot.
굿바이 썸머 (Goodbye Summer) is about a high school boy deals with his terminal illness decides to live his remaining time back to being a student and falls in love with a girl.

Jung Jae Won known as One and Kim Bora is such indeed a visual pair in this film but too bad it wasn't fleshed out in which they have this little interaction that you can't tell whether they really like each other.
Honestly, this film is not about romance nor neither about the fragrance of youth but a film about a person's life and his perspective towards his surroundings & to the people he interacts with his limited time.
The dialogues are engaged in a humanely way but at the same time incorporating the age of the characters.
There's something dull about this film that is not very engaging.

Back to the song that I mentioned:
The feelings I’ve hid still remain as a painful secret memory
-This depicts Hyun Jae's feelings for Soo Min, he could never say what he really feels thus their memory together is a painful secret memory for Soo Min and Hyun Jae's affection for her will never be known by the world.
The photos that can’t define our relationship is a heartbreaking story
-This line could talk about Hyun Jae's & Soo Min's relationship because they never really knew what each other's feeling; that they can't define what kind of relationship what they had which is a heartbreaking story for the viewers.
I’m sorry, summer, now goodbye
-Hyun Jae making memories with Soo Min on the flashback is a summer story for him that he eventually has to say goodbye.

*contains spoiler* In the final act, there was this moment between Hyun Jae and Soo Min. They are talking inside the classroom, probably Hyun Jae asking her out while Soo Min knew his condition; they have this little talk about selfishness & feeling guilty.
That interaction between them but mostly Soo Min who poured out her emotions somehow gave you an idea that she indeed fell in love with him in the process but for Soo Min, Hyun Jae's actions were taken as being selfish but for Hyun Jae he somehow feels like he asked her out with sincerity of adoration to her.
You can see the two weights of it, Hyun Jae doing what he wants with sincerity but people around him keep reminding him that he needs to think things through because of his limited time while Soo Min has that long term thoughts about what if someday when he's gone, she will suffer by it because she fell in love with Hyun Jae.
Both have different descriptions about love but they felt the same way. It's sad because it never really push to the positive side of falling.

I thought I'm going to cry about this film apparently not; There are just moments of sentimental and hopes that it could have been more but never really expanded especially the climax & conflict.