Why is nothing ever as good until it's gone?

Why is it that we don't appreciate what we have until we've lost it?

People say hindsight is 20/20. But is it really? Think about it. Maybe our minds remember things differently than they actually were. Maybe we glamourize past relationships, jobs, events. Maybe we remember only what we want to remember, making it all the more difficult to forget.

Do you ever miss someone from your past? Do you ever sit there and feel such emptiness because they're no longer beside you? But if you really think about it, when they were beside you, were they really that great? Or did they just annoy you?

This is something that happens to me often. When I'm immersed in a relationship, every little thing my significant other does makes me want to kill them. But after I've had enough and decide to leave, I find myself missing them terribly. Is it simply because I'm so accustomed to their presence in my life, or do I wear rose coloured glasses when peering into the past?