But the fact is, she was the first who came towads Joonwoo. I think joonwoo love Sungah but not a "true" love, let me explain, we know that MC is quite asocial (to not saying fully asocial) he had no friend, or no one next to him to talk etc, when someone get interest in you to being friends, the MC imagine, think, look this act like a savior act, he thinks that this person bring full attention on him, that maybe has feelings, something that may the others "random guys" have not towards him. I think that's happening or happened, he loves Sungah coz she was the only one who saved him from solitude/bullying etc, loving more like a psychological savior and keep having admiration than a real love.
It's the same when you are in a deep black hole, being pessimistic, depressive, and someone arriving and takes you out of this state of depression, you need consideration, you need regard, and when someone give you that, you make lot of scenarios etc
Doing some psychology studies by my side, Joonwoo could possibly be at this state of thinking