I believe scientists still don’t know how we cry, but I can give you an example as to what causes the event. When you tickle someone who’s ticklish, they laugh not just because they are happy, but because the nerves react to the touch differently than, say, a pinch or a punch. The information sent to the brain is interpreted as “This is a touch that is not harmful nor usual,” and that unusual feeling can cause the output of a laugh or giggle or a riotous drunk-man collapse of joy. The reason some people cry is that their brains believe the touch is starting to become more threatening or imminently dangerous. This can happen if a person is being tickled too much, or if someone is extremely sensitive to a tickle. That threat of pain goes through that unexplainable process and the person begins to tear up. Be warned, though, that this is my best guess and is not proven by any written or otherwise exterior source. Of course, it should be somewhat correct, as I am extremely ticklish.