What is to laugh or cry? What is going on inside our bodies during these processes, both biochemically and in our central nervous system? Why do humans do both while no other animal on the planet does either (at least not to display happiness and sadness)? Why does laughter feel good?

I've been intrigued about this for a while now; I recently saw a guy crying on TV, and I nearly burst into tears myself with such heart-wrenching sympathy and sadness. I couldn't explain it. I can't remember the last time I cried but this man brought me to the edge. Once I started questioning the act of crying I began thinking about laughter as well.

So, who can answer my questions, or add anything else?