Well, who wouldn't have "Second Lead Syndrome" when the lead looks like cr*p.
There's nothing mysterious watching someone who realy isn't a looker and is clearly too old to be a student, judging from the deep lines around his mouth (yeah, the laugh lines... his life must have been SO funny), and after a while a young and cute boy appears. Yeah, do you wonder that you're having "syndrome" lol.
Besides Seo Kang Joon there is also Nam Joo Hyuk so basically everyone around beats the "lead" guy in this.