For me it's a little bit of both.
I consider tickling to be fun and playful. Sometimes it's harmless fun, but other times I find it strangely arousing.
As an example, I used to tickle my female cousins relentlessly. Never once did it evoke a sexual response from me. It was just fun to watch them laugh their heads off at the wiggling of my merciless fingers. 😛
In the same light though, when I used to tickle my ex, there was a very real sexual component to it. Or when I would tickle, or be tickled by my best friend, who is a female. We used to get into tickle fights sometimes and I can't deny that I became aroused as a result.
As I'm sure it's the same with most people, it all depends on the context of the situation.