Harry is afraid that he might be lulled if he gives in to some of the temptations. It isn't as literal as believing that Lucius would say, "Ha, you slept in our bedroom, now you're our slave!" It's more that Harry thinks he might get used to these things and come to miss them so much when they're taken away that he'd give up some of his moral principles for them.

And, well, Harry has less faith in himself than he used to, since failing the Potions test.

He also thinks that Healers (better at Potions than he is, and also with more knowledge of healing magic) could perform much better spells, and that the Malfoys are placing a little too much stress on something that could have happened with anyone taking care of Lucius who knew how to perform the spell. He considers the action from a mediwizard's perspective, and that only. And now he has to wonder if they value him for what he did and not who he is- like almost anyone thinking about him as the Boy-Who-Lived and the Defeater of Voldemort.

And thank you!