You don't need a reason, but the development doesn't happen out of nowhere. They were both like "I hate xx and I'll destroy his life" toward each other and bam the next episode they started courting each other. A good writer would show us this gradual development from hate to love, it didn't happen in this one as it was far to sudden and jarring.
I'm actually excited about this despite the recast. I think these two look really good together.I wish their characters were more explored in Cutie Pie.
I really wish Cdramas were shorter. Seriously most of them would actually be fun to watch if they actually didn't drag for so long for no reason. Like this one was sooo painfully dragged. The main story could've been done within just 15 episodes. Cdrama producers need to understand quantity doesn't equal quality.