And he saw them dancing

He should have seen it coming when Draco's obsession with that girl became more than curiosity over her brilliance, more than hatred over her blood, more than incredulity over her damned beauty.

Lucius was a smart man, and he should have seen it sooner; that every time Draco referred to her as 'Potter’s filthy little Mudblood', he was only covering up for how in love with her he was. He should have seen it, but he ignored the signs until it was too late.