Because of what OtherHandle said, basically. Yes, it can be a dangerous and self-serving attitude that you use merely to rationalize your abuse of another person, but I also find there to be something refreshingly honest about it, at least if you're open about it (which Harry admittedly wasn't).

Reading that line caused me and a friend to have a very pleasant conversation of the ways in which we are going to use each other for our mutual benefit in the future. :)