The first thing that caught my interest (beyond a certain desire of dominating the Boy Who Lived as such) was the way he stood up to Snape - his strength, and overwhelming will to survive. And later on, seeing him with my son. I was able to control and mould him in a way I could not do with Draco, who is a Malfoy, not a tool after all. I saw his immense potential in magic and personality. And finally, the way he responded to me, perhaps against his better interest - he *wanted* to be loved, wanted someone to care about him more than anything, no matter the price. And I could do that for him, and use him in return.