...because it's not normal love, as I said it before...
Love has a lot of forms and saying obsession is not love is just dumb, since love itself is a psychological obsession at the very first place. ou just don't want to be az object of this love... Just think about it, what love a little kid has to their parents? It's an instict, a tool for a survival that trigger another instinct (even for strangers sometimes!!) that provide caring, food, protection, and everythingelse for the kid, for a lifetime, if it's necessary. And we call it "mother's love'. A pure obsession.
In a family we are obsessed with each other for a long-long time, to the point every (okay, not every, but an average) parents thinks their kids are the best, the most beautiful, the most clever, the most talented, the most valuable of all kids, and even if they not, they just literally don't care, because "my kid is my kid, and that's different!" That's also pure obsession.
So yes, I can just repeat myself - Sangwoo loved B**. Not necessary a romantic, or even a "normal" way, but he did. Just like he loved his mother. And I mean, literally like his mother, a totally uncontrollable, ficked up way where fear, anger, hatred, obsession, several complexes, etc are involved.
I really can't explain it more, it's.. It's like when Ted Bundy said "That won't happen to her' when his girlfriend worried that the mysterious serial killer attacks his daughter one day. That won't happen, never. Not to "his family."