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Let me suggest that lightheartedness is not a character trait but a response to circumstance. Let me suggest that a lighthearted character is one who expects to get the things they desire, and that a heavyhearted character is one who does not expect to get the things they desire. 

A man who goes courting expecting the lady to say yes, sets out about his task lighthearted. 

A man who returns to his home expecting to find his family all dead, sets out upon his journey with a heavy heart. 

It is true, of course, that some people seem to have a naturally sunny disposition, and that others seem naturally dour. Perhaps that is a psychological preset. Perhaps it is the expectation of life that have been formed in them by prior experience. So their character may condition whether they approach a project with the expectation of success or failure. But still, one can see their heaviness or lightness of heart as an expression of that expectation. 

In other words, perhaps what you have to do to to create more lighthearted characters is to create a plot in which the protagonist fully expects to achieve their desire. 

It won't be that easy, of course. The rain must fall sometime, or there will be no story. But if there is no story if the sun shines all the time, similarly there is no story if it rains all the time. There must be sunshine after rain. These things have always been the same. Why worry now?