My interpretation is that the emotions experienced prompt the laughter, rather than the laughter being a behaviour in itself. We laugh when we are really pleased (glee) and when we are being cruel at others' expense (scorn). As far as the 'giddiness' is concerned, it refers to the childlike happiness that we get when everything is wildly exciting, perhaps reminiscent of feeling giddy (dizzy) and out-of-control.
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