Lucerys Velaryon died at a young age so the angels thought of a new way for the poor boy, he was reincarnated as Alec Lightwood. The mundanes have no idea about the shadow world, they were all caught up in there battle that they had no idea about the Downworlders and shadowhunters. The angels wanted him to have a new life, they thought he had potential to be a shadowhunter. And if he falls in love what's wrong with that?