I haven’t seen this series on my own so my speculations r purely just speculations lol but imo I think both jaemin and soohyun knows daon has a crush on jaemin, therefore when jaemin said “you like daon right?” to soohyun and followed up with “if we agree to date each other I won’t date daon” so soohyun agreed cause he knows he can’t win ? jaemin though I have no idea what motive he’s got 😭