I can't think of any overall great reason, just a lot of small reasons kind of fell into place.
1. I've always had a thing for Harley Quinn. I remember watching Batman when I was seven and thinking she was just the coolest person (all but the battered women's syndome, but that's the Joker for you
). I figured if the Joker had the joker playing card, then Harley would be best represented by the QoH.
2. I used to play Texas Hold'em, and the Queen of Hearts was instrumental in me winning a fair amount ($500+). I ended up hitting a straight flush to the queen. To this day, that is the best hand and most won off a single hand I've ever had (although to be fair, I don't play poker any more, so those records will likely retire). Since winning that, the QoH card has become a good luck symbol for me.
3. I liked the sound of it as a screen name when it came to me and was very surprised no one had taken it.
4. I love the band Queen.
5. I love heart designs and like the band Heart
6. To an extent, I enjoy people lusting for me. I find it an affirmation of my femininity (i.e. I enjoy being the Queen of their heart).
7. Most importantly, I feel like its a somewhat unique screen name which accurately represents me.