This is random, but my husband's laughter shares some traits with his crying - similar vocalizations, belly hitching, Adam's apple bobbing. Of course, it sounds like laughter, but if he's laughing really hard, my mind sometimes makes the connection. I noticed this when we were lying in bed, and I had my head on his stomach - he started to laugh, but it was so similar to his sobs that it started to turn me on.
This got me thinking about similarities between laughing and crying, and I'm wondering if everyone's laughter and tears share some common traits. I think my laughter is somewhat similar in pitch to my sobs, but I actually inhale much more sharply after laughing hard than I do after even very intense sobs. I don't notice the subtleties of laughter nearly as much, though I was also thinking that anyone with a laughter fetish is lucky to get much more frequent observations!