Why is first love so unforgettable?

Because of the happiness, the hurt, and the regrets after.

This is a classic campus love storyline. 

I like the fusion of classical and modern instruments. 

I like how Lee Sin reject girls' confessions by saying they are ugly.

I like the band name The Stupid. 

I like how "first love" is being portrayed. 

First love in progress: Lee Sin & Lee Gyu Won

First love aft 10 years: Performance Director & Professor

First love after 20 years: Lee Sin's mother & Lee Gyu Won's father

From Lee Sin & Lee Gyu Won, it's not difficult to relate to the "first love" of other characters. 

The professor said to Gyu Won, that she never regretted breaking up with her first love to pursue her dreams, although her dreams were eventually crushed by an injury. And that, is youth. 

Lee Gyu Won's father told Lee Sin to initiate the move, because one of them has to do it, or else, they will end up like their parents, life full of regrets. 

I like the song Lee Sin played over the campus broadcast, to ask her to smile, that everything is alright, and he'll always be there. 

Too many scenes that pulled the strings of my heart. 

Highly recommend Heartstrings!!