... more only when we can no longer have?
That is because we have no idea about anything. As much as we think we do, we don't.
Say, you walk on a street and you see a BMW. You like it so you tell your friend: "What an awesome car. I wish I had one!" He might say: "You know, Mercedes is a better car, I wish I had a Mercedes car instead."
Now who is right? Of course, none, both are looking at the things from his own perspective.
Say you have a good friend for 20 years. He is your best buddy in the whole world. Then you find out he cheated you with $100. You get so mad that you never speak with him again. "What a crook!" you think. When were you right, 10 minutes ago when you liked your friend or now when you think he is a thief. It is the same person, you know.
Same about memory. Try to describe the person you saw when you came out of the house. Probably you don't even remember him or her. The memory is so weak so when you remember something there is a bigger chance is wrong than right.
And now coming to talk about people or things you don't appreciate until you lose them. The reason is that again you have no idea about anything. All that you have is your image about that particular thing. It is an artificial image, lots of time with no relation with the reality. An image your brain is building using the thoughts as tools. And such bad tools the thoughts are. They can build a monster out of a saint and the other way round.
When we have something, lots of times, out thoughts are making a mess out of him or her. We see the bad habits and we amplify them using our brain.
When we lose somebody, we start to see what we don't have anymore and then the better side of that person or thing comes into mind. And using our continuous though process we build a statue out of it.
(More Q&A with Satty here. If you have a question, send it over.)