Well, let me give this answer.
I saw a sex talk type show, and there was an expert on. A doctor. Someone from the audience asked about fetishes, and what causes them.
The doctor said that there was no real way to discover what caused a fetish, that there could be any number of causes. He also went on to say that while many people enjoy some time of "fetish behavior" that the true definition of a fetish is that the person with it can not achieve orgasm or errection without the behavior being there.
In other words, members here may enjoy tickling their significant others, however, for it to be truly a "fetish" you can not achieve any type of sexual gratification with that significant other, unless tickling is involved.