I was expecting to see quirky stuff when I started looking at this thread. I am disappoint. So in that spirit, I'm just going to start listing some of the reasons that my love interest loves me back.
1. I lavish her with attention. This might contradict what SavingPrincess told everyone, and normally he'd be correct in how to get a girl, but she's different. Her self-esteem is just a tad low (she doesn't think she's exceptionally beautiful, but is confident enough not to care) so I shower her with compliments. Which I love to do =D
2. I can crush a man/woman's soul. Yes, this turns her on. She enjoys the fact that when I get mad, I can utterly destroy a human being's life through harassment. Words are powerful and I am their Wordsmith!
3. I'm good at video games. I don't mean "I play" video games. I mean, I want to beat the shit out of this video game and nothing will stop me! She can find it intimidating at times, but it's her favorite passtime and likes that I'm just as passionate about them as she is. That and the fact we live in two seperate countries at the moment means we can still have "dates" by playing something online. This also has the added benefit of playing into reason #1 listed, where I'll spend countless hours getting some pointless item she'll like, on my own accord, and regale her with the effort I went through to get it. She finds it sweet.
4. I'm silly. This is how she puts it. I'm not talking about the "he makes me laugh" or "he's funny", I'm talking about the kind of silly that makes you feel like you're four years-old again. But who doesn't like being four again!? My imagination is endless and she loves watching it all unfurl around her when I decide to let it out and roam free for a bit. It also means that I'm not afraid to say I like childish things, like play Pokemon games, which are apparently taboo where she lives right now, so it makes her feel that much more normal.
5. I'm a gallant. Not the way she would word it, but I put a few of her reasons together. I'm always there, with no exception. She knows I'd go to the ends of the Earth and back for a single flower if she asked for it. She knows I'm polite, curtious and a bit 17th century Victorian in my manners (yes, I kiss hands still). I obey the law, even if it does seem unjust at the time and right wrongs at every turn I can. I'm determined to my cause and I don't back down just because I hear "no". My determination to follow my heart and soul are matched only by my ingenuity to solve problems when my confidence and speech will not. In short, I am her over-zealous knight in shinning armour, a beacon of light amongst mankind, and overall pinicle of righteousness. How else would I protect my reserved little princess from the horrors of this world? =D
Hope you enjoyed my list. Here's to sillier answers in the future for this thread.