Laughter can be dreadful

Remember the song in Mary Poppins about laughing? I can't figure out how to post this video, so here: http://www.youtube.com/watch?v=gx7lz5X2vKk I know a person with many annoying laughs. Let's call her Jane. Jane sometimes hisses like a snake. ssssssssssssssssssssss Other times, Jane wheezes like she's going to have a heart attack. Which gives me a heart attack, because I naturally assume I'm about to see someone clutch their chest and fall over. Frightening. She will even cackle on occasion. I don't think I've ever cackled. What can come over a person to make them cackle? I don't understand that emotion. Why is laughter, a positive thing, sometimes so annoying?!