When things are well its easy to be happy. But after a while, it becomes status quo and you start wanting more. It's easy to forget how well things are and start looking forward. Of course, there is nothing wrong trying to improve what you have, that's a great driving force. But constantly looking forward can easily lead to unhappiness.
Looking back its also not always apparent how bad things were back then. It might be that everything has been relatively well forever. There has just been some gradual improvement over time, but nothing dramatic to compare against.
But what if you'd lose everything you have today. Just all of the sudden it would all be gone. What would you lose, what would it be that you would regret the most? Imagining that makes you really appreciate where you are today. It's not about gaining more, its about retaining what you have today. And when you realize that you will be a lot happier.
Just remember its all hypothetical. Don't fret too much on the possibility of losing all those things. It could lead to unnecessary fear of loss. It's a thin line, but expecting the worst can make you happier.