The are two types of tickled reactions one of of which is from soft contact such as the feeling of having an insect crawl on your arm, and the other is the hard type of tickling where we twitch and can't help but laugh. Most scientists agree that the former evolved to enable us to notice when potentially dangerous insects are on us, the latter however there is much debate as to why we have are ticklish in this way. It appears to exist for social reasons and also seems to be limited to primates. The sensation arises from the stimulation of nerve fibres associated with pain and touch. The areas in which we have the most nerve endings are the most ticklish.
The jury is still out as to why we cry, there are two main theories, the first is that it is merely an autonomic response to pain, the second is that it is a nonverbal communication technique to ellicit help from others.