I know it doesn't make much sense, but Harry thought that, since Draco manipulates other people so much, he wouldn't mind as much if he were manipulated himself. He might even appreciate it, in some odd Slytherin way.

But mostly, he really wants to be involved in Draco's life somehow, and he knows/"knows" it's never going to be the way he wants it to be, so he's making up excuses so he can go on being involved.