According to Osho, laughter and tears are two faces of the same energy; some are 'laughter people' (extroverts) whose overflow appears as laughter, others are 'tear people' (introverts). So you laugh because that's your natural polarity—don’t imitate tears. Go totally into your own way; at its utter extreme, laughter will naturally transform into tears, and truth is revealed.
You laugh instead of cry because that’s how your feelings flow; let it be, and if you go fully into real laughter, tears may come by themselves.