Just like pain or sadness, laughter affects the part of the brain that controls body functions. In this case, a signal is sent to the lacrimal glands, they begin to produce fluid - and we cry. Scientists suggest we shed tears to show how we feel. That is, tears are a sign that we are overwhelmed by strong feelings.
Paul Ekman, in his book The Psychology of Emotions, argues that tears are the most versatile way of expressing emotions. Any emotion, if it is very strong, can be accompanied by tears. I think this is a kind of physiological reaction that helps the body to distribute emotion and "give" part of it from the psychological level to the physiological one.