Well, I have read the novel and I do agree that the obsession starts because of that, and while I do agree with him having no choice but to keep Hohyun alive for him to keep going, he was actually the one to notice hohyun first in the first round, and from there, i would say the obsession and interest aka falling in love goes hand in hand and the line between them is blurred. Just like Hohyun's feeling. While he is gradually falling, by the time he knows about the regression, the sorry and regrets intensified and that causes him to be more accepting towards sunbae. The line between love and sorry is blurred and I'm not sure which is stronger. In the end, they both become obsessed with each other.
But what I can truly be sure that he is indeed in love with Hohyun and not just a mere obsession and vice versa is when they have get out from there and have a normal life together.

So by what im reading seme has become obsess with uke bc he had no choice and if uke dies all starts all over again. that why uke knows nothing and why seme knows everything that will happen and that's why he can easily get all those things without suffering any consequences