【Lightness】 , 【light-mindedness】 ,  【levity】 ,  【frivolity】 ,  【flippancy】 ,  【volatility】 ,  【flightiness】  are comparable when denoting the quality, manner, or attitude of one who is irresponsibly gay or indifferent especially when seriousness is expected.

【Lightness】 implies a general lack of weight or seriousness (as in character, mood, conduct, or speech). The term may further imply instability or carefree heedlessness or indifference to the seriousness of a situation.

【Light-mindedness】 , even more than  【lightness】 , suggests a temperamental lack of seriousness or stability.

【Levity】 usually suggests more specifically trifling or unseasonable gaiety.

【Frivolity】  adds to  【lightness】 the implication of empty or idle speech or conduct; the term often carries a strong connotation of triviality or of pettiness, but its most frequent implication is that of such indulgence in meaningless gaieties that serious employments are disregarded.

【Flippancy】 applies especially to unbecoming 【levity】 or pertness in speaking of or in dealing with serious things.

【Volatility】 implies such 【lightness】 or fickleness of disposition as precludes long or serious dwelling upon any one idea or plan.

【Flightiness】 may imply extreme 【volatility】 , often with a suggestion of loss of mental balance, but it often suggests extreme capriciousness or a gay whimsicality characteristic of one who is not long contented with what he has or does.