I guess this could simply be a term used to describe appropriate, useful mourning. There are times when a person mourns when it really isn't warranted. I think appropriate times to mourn include:

- the death of a loved person, or creature
- the destruction of some object that had a sort of sentimental value to you

I also think, even though it can be a beautiful thing, it is sometimes appropriate to mourn the the setting of the sun. If you've had a particularly excellent day, and it's coming to an end, maybe it's alright to wish that it weren't, and maybe even be sad. This should be a relatively brief mourning period, though, because life goes on, and the coming day holds infinite amounts of potential, it just needs to be used correctly.