Both ido and sejin were the victim of the situations they were in......sejin wants his family's recognition desperately, no matter what they cost him ( he did steal the papers thinking ido doesn't care about him so he also does'nt have to care about him) and ido was a born superior and was used to seeing other people trying to take advantage of him that's why he can't comprehend sejin's kindness without any motive+he dealt with his adoptive father so he thought both of them are in kahoots especially after sejin said he stole it willingly for his family....rest things spiraled out in worst way possible and that's how their first life ended....but with a new start ido did change himself and sejin although with time did change his attitude of blaming himself for anything wrong happening around him. But this is what make them seem more human.....we humans are not born with sixth sense to judge other's thought's, we will make mistake but we also try to mend it when we realise.....i just hope both of them can cross this hurdle and live happily in peace together