I hate being tickled. It is the only thing that can entirely ruin the experience of laughter. It feels too out of control for me, which is slightly hypocritical because it's great to participate in side-splitting laughter otherwise.
I just don't like that one person ultimately has me so conveniently disabled. It's not fun for me to be tickled, I put all my efforts into stopping the person tickling from continuing to do it at any cost while restraining the laughter. Which puts me at odds with my girlfriend, because she enjoys tickling me with the chance that I'll laugh. Sure, I tickle her, but she at least enjoys it and stops laughing afterwards with a huge grin on her face.