mutal breakup; when a couple break up because of a particular reason, they break up together not one breaking up with the other.

Reminiscing the past when they were lovers.

but now they're noona and dongsaeng.

that one choice they both did not want to make.

They secretly still love eachother,

they care about eachother, they worry about eachother, they think about eachother. 

but they know that they cant be together, it would make everything worse.

so they reminisce the past, the memories they shared with eachother.