Continental philosophy seems to use words as much to trigger reactions and emotions as to relate concepts, so I have my own loose thinking about lightness. I see it as having two overlapping aspects: a strong sense of one's ability to choose freely and go one's own way (i.e, one is not excessively weighed down by prior commitments and expectations); and a view of life that is not overly serious, that takes things as they come, that eschews drama. Of course, there is necessarily weight to one's choices - some of them affect the rest of one's life, and we have to live with those consequences. Thus a radical lightness is contradictory in both aspects - if it leads to carelessness, we create drama; if we are obsessively committed to it, we have constrained our freedom to choose.
Lightness is a way to counteract the inevitable heaviness that daily life imposes. A smile, a change of scenery, a different perspective, a letting go of attachments. Life does not need to be incessantly heavy, or filled with anxiety or dread. Just live it.
And that brings me to Jazz. An epiphany emerged today that Jazz is an aesthetic embodiment of lightness, in both of the aspects mentioned above. First, it is a generally lighthearted, happy genre - even the occasional deep or painful topic is smoothed over with a lilting saxophone or clarinet - and mostly it just sounds like a good mood. Second, the technique of improvisation is central to Jazz. Of course, musicians sometimes improvise in the solo in classical and rock & roll, but this is generally viewed as an aside, and in any case it is relatively constrained. Improvisation in Jazz is precisely a lightness, a disclosure of immediate and intuitive choices that are what the musician knows and feels at that moment, and this comes through to the listener.