
Spoiler:
He’s not really shown suffering, it’s basically Taekjoo finally gets away, then things happen and they meet again and at that point Taekjoo is like “what the hell, you’re gonna chase me forever, so it’ll be fun to try and tame you.”
As for when or how he fell in love, it was gradual, but he didn’t realize what he was feeling. After he kidnaps Taekjoo, they spend a lot of time together and things slowly start to calm down. At some point he asks Taekjoo why he keeps trying to run away and Taekjoo tells he he has no reason to stay, to which Zhenya replied “find one”.
Part 2 finally answers some questions from part 1, though, for example, at the end of part 1, when they meet again, Zhenya says Taekjoo picked him first, which Takejoo was like “huh?, when?” Finally in part two we find out that he took Taekjoo’s initial interest in being partners as something else (it’s mentioned a lot in the novels that Zhenya has the tendency of interpreting things differently than intended).
Also in part 2, the plot involves Taekjoo losing his memories, so when he remembers, it’s not pretty because he only remembered up to the events going on right now and he’s having trouble believing he actually willingly chose to be with Zhenya (at this point they’ve been dating for 3 years and Zhenya is bffs with Taekjoo’s mom). He’s pissed, thinks he must’ve been coerced, or had given up, or something, and we get a bit of Zhenya’s thoughts about that time. It says that originally he found him amusing and was suppose to be a passing entertainment, but when Taekjoo went after him again, his interest piqued and at some point he just wanted to be with him.
Seems like it's better to read it after Zenya starts his own suffering...... But can someone spoil me what will happen later and how did Zenya fall in love eith Taekjoo?