I've looked it up in several dictionaries and I've got that the meaning is more or less similar to "cheerful", "carefree" etc. What is not clear enough to me is this point: is someone defined "lighthearted" about a certain situation when they are cheerful because they have no reason to be worried or concerned in that respect, or when they just don't care about that situation (regardless of any possible problem) because they are superficial and irresponsible?
I'll provide an example to clarify what I mean. If someone says "I'm not so lighthearted about my marriage", what does they mean?
- There is something in their marriage they are worried about.
- They take their marriage seriously and don't make light of it.