I'm coming super late to the kdrama land. I've watched several kdramas and mostly enjoyed them but never to a point where I actually browse the internet to find out more. Reply 1988 is the first to make me realize that kdrama can also offer some depths.
Anyway, I have to chirp in for this. I think Taek has a more layered feelings in regard to this love-triangle. His first instinct when he realizes JH likes DS is to immediately ask her for a date. But the more he thinks of it and even more so after he sees the photograph, he realizes he can't go to DS if it means hurting JH.
JH, on the other hand, immediately lets go of DS when TK says he likes her. At this point, although TK is stronger candidate, nobody 'wins'.
The writer and director repeatedly purposely shows the subtle difference between TK's and JH's feelings toward DS.
After JH decides to stop pursuing DS, she pretended her foot hurt but JH was reluctant to even support her walking although DS repeatedly asked him to. Despite already decides to stop going after DS, when she can't walk, TK carries her, princess-style, after DS declines his offer to carry her on his back.
The peak is in ep 18, when it is TK -- who doesn't even know how to get to the concert venue, gets lost easily, can't even park his car -- who arrives first. JH is left again, brooding until he finds out that TK forfeits his game because he can't stand the thought of DS going to the concert alone.
This brings back to Bora's narration: "To love is not just wanting to give but a desperate desire to give." Then JH remarks: "The punk is more desperate and I should have been more courageous."
Game over. JH is clearly defeated.
Also it's been established that JH's love for DS is that type of high school love, a first love that hasn't matured yet and focused solely on himself. Remember when he learns SW 'rejects' DS? DS is heartbroken and ashamed but JH laughs, even gives SW sympathetic pat when DS is clearly still mad at SW. Years later, after TK reads DS' diary entry and finds out how SW breaks her heart, TK hits SW hard on the back.
JH's love for TK, though, is a more mature one. Bora narrations: "Loving someone means you can't hate them." Compare JH reactions when he thinks SW likes DS to when he knows TK likes DS.