Probably above my pay grade, but as I see it the issue is how we typically relate to the world that causes this kind of sadness, rather than the fact itself. It's like, even though I consciously understand nothing lasts forever, I can't help being sad by it. Some deep structure of my mind won't accept it. The Buddhist aim of insight meditation is, as I understand it, to get to a deep enough level of your mind that you finally accept this at your core. This brings about freedom/liberation, as from then on you're able to just go with the flow without the slightest desire for permanence (or anything really). At least, that's the sales pitch.