A lot of people who write BDSM in this fandom have never experienced it in real life. Or perhaps they've played with kink, but not with actual D/s and hardcore S&M. There's a lot of psychology there (as I'm guessing you might know), and really, Harry would be a prime candidate for needing a cathartic experience through S&M as a bottom. The more stoic the person, the more hell he's been through... the more he might need that chance to let go, and really need a *push* to get him there. :)
I'm glad you enjoyed it. (Of course, I should also mention that my recipient requested bottom!Harry, but I actually prefer top!Harry in my H/D.)