When little Lily eagerly promised her friend Severus that she would marry him and love him forever, little did she know that magic heard her words and felt the truth ringing in them at the time. So the magic took and run through Lily's blood. When Lily didn't fulfill her promise before her death, the magic dormant in the only blood that remembered that promise started to stir; and once the seal that Voldemort's brand on Harry was removed nothing stood in its way any longer.