There can be different reasons to be sad for a long gone past: * You were sad in the past and you're remembering being sad * You were happy in the past and yo

There can be different reasons to be sad for a long gone past: * You were sad in the past and you're remembering being sad * You were happy in the past and you're a bit "sad" this time has gone or that you forgot what it was like to be X etc I believe melancholy is the latter. You're not truly sad because you're also enjoying a nice memory but at the same time you're also a bit sad it's gone (hence the "happiness of being sad")