We cry when we're sad but why do we cry when we're overjoyed by something? Being sad and happy are two complete different thing. But we're also very emotional when we're very happy and very sad. Crying is a intense emotional response. When feelings overwhelm us, they show as tears.
We cry because when we laugh we put pressure on our tear ducts, so it forces the tears to come out. Or in more biological way tears also accompany the body's return to homeostasis after extreme excitation. So after a big laughing jag, tears are a sign that the body is returning to normal.