That's an interesting question. I don't think that anybody really knows the answer to that question. I can imagine that being aware of your environment (for example, poisonous spiders crawling on you) would be important evolutionarily. That ticklish feeling makes you more alert and causes you to move your body away from the sensation. Why it would then make you laugh ... I'm not sure how that would come about. Perhaps this feeling stayed with us since it can be kind of a social interaction as well. Typically another person tickles you and usually that other person is someone you know. So perhaps it is a social thing as well, something that enhances the connection between people. Of course it could just be some weird tick in our systems that was never bred out of us since it's not really hurting us.