If I delve deep into my mind and really try to analyze what it is about tickling that turns me on, I find that it might be the fact that when you're BEING tickled, you're not in control. The other person can make you laugh hysterically, or simply giggle, or maybe not laugh at all. Naturally a submissive person, the fact that being tickled is (at it's most basic level) a loss/surrendering of control, it doesn't surprise me that it's something I'm aroused by.