I'm trying to picture the most likely scenario where this would happen, and in my mind it goes something like this:
I'm at the mall, because I really need a new pair of sunglasses. As I wander over to the stand on which sits many pairs of ugly, ill-fitting sunglasses, I notice a blonde teenage girl walking in my direction. Maybe she's 18, maybe not, so I make little note of her. After all, I don't want to be a creepy dude in the mall. Since people constantly pass by me in the mall, this is not much of an event.
However, when you pass me by you tickle me, perhaps by goosing my sides. I jump equally from surprise and ticklishness, but wouldn't laugh. Instead, I'd be shocked and concerned. I generally don't want random strangers touching me, especially in a way designed to get a reaction from me. If you kept at it, I would first try to back away from you, then confront you about it and get angry. It would not be a pleasant experience. I'd wonder where your friends are hiding as they giggle about how they dared you to do this.
But... I don't think this is what you were going for. Let me try to picture something closer to what you seem to want:
You and I are about the same age, and you are definitely over 18. We are friends, lovers, or something in between. We are in your bedroom, idly chatting and enjoying each other's company. In response to a sarcastic remark I made, you reach out and goose my sides. Now, I am secretly a little thrilled at the opportunity to tickle you back. I grin and retaliate, skittering my fingers over your sides and belly. Instead of relenting, you giggle cutely and keep trying to ineffectively fight back. This only encourages me, and I continue to tickle, wriggling my fingers underneath your shirt to tease your bare skin. This continues for a few minutes until we reach an armistice, lying there panting and red faced. This intimate moment may or may not lead to sexy goings on, depending on the parameters specified above.
So... either of those, depending on the premises and what you're going for with this thread. I'm easy.