Here's a weird one:
Let's say that for whatever happenstance, you get the one in a lifetime chance to alter the events of the past in a person's life, with the sole objective of giving said person a tickle fetish. You can name any situation or ocurrence that is within the boundaries of reality and it will have happened to them, at any point of their life that you choose. If you word it carefully, it might even be something that occurs during a prolonged period of time, but you cannot change anything physical in this person for an easy win (no magic: if they're tickled they get aroused answer). If you succeed, and the person indeed ends with a tickle fetish, you are guaranteed to form a connection with this person through this fetish that will result in a fruitful relationship where the both of you enjoy tickling and other romantic activities for a long time, possibly all your life (even if you haven't met this person, this guarantees you will). But if you fail, that person will have zero interest in tickling as a fetish with no chance of changing that and, in addition, will have no interest in having any sort of relationship with you, although this does have a chance of changing (it would take some effort).
So! What would you do? Would you take the chance? Who would you choose? And more importantly, what situation could you possibly put in a person's life that would guarantee they end up a tickle fag like us?