Well my opinion is that the tickling fetish is just like any other fetish: It's a sexual fascination with something other than sex with no reason in particular to elaborate. However, I personally believe that the tickling fetish is something that is a bit more...sensical? I don't want to offend, because I believe that all fetishs are fine.
But in the tickling fetish, it involves people touching, stimulating, and playing with the nerve endings on our body. In a way, it's like giving people (or at least people who are into it) small orgasms over their body (sounds cheesy, I know, but stick with me). The physical touch of another has always been looked up at if it's a good feeling, and in tickling it invokes a pleasurable response both mentally and physically.
I don't know, but it seems like tickling fetishes have much more of a reason behind them than, say, a fetish for inanimate objects.