It's the whole thing about life being a roller-coaster. It's full of phases, ups and downs.
If good things were the only thing that ever happened to us, we wouldn't appreciate them as much.
But the plus side of this is that when things aren't going so great, you can only look up.
It can take time to work out the balance of it all but when things are going well, that's when it's most important to think of things in the present tense. Don't worry so much about the good moment passing, just focus on enjoying it whilst it lasts. Then, if it does pass, you can still feel buoyed from knowing that you made the most of it, and that things can be good; something people sometimes forget in times of sadness.