Wanting what you can't have, and not appreciating what you do have in life.
I don't feel that way as much as I used to, for me, the feeling came from trying to fill up the restlessness in my heart. I'd view someone or something as perfect, or rather "perfect for me" and subconsciously believe, "This person is going to complete me, somehow." Of course that's unrealistic. People aren't perfect, and no one can complete another human being.
You, me, everyone - we're complete to begin with. Anything to the contrary is an illusion. I'm pretty much over the idea that material possessions will ever provide peace inside my heart and mind. And from experience, I know the ONLY way to peace and happiness comes from inside yourself.
I've come to realize the possessions or people we desire aren't with us, because they aren't going to help us get to where we need to be in life. God makes it difficult for a reason. The stars don't align with the idea of what you're wanting because it's probably not what's best for you.
Here are a few key indications of something not being right for you to think about:
1. You question yourself. ex) Should I go see this person or not? Do you want it for the right reasons or for selfish reasons?
2. If everything works against you, and what you want. ex) No matter what you do, something always prevents you from being with this person or making a certain deadline.
3. Your priorities shift. ex) You no longer put yourself first.
Allowing everything to fall into place naturally, you'll reap great benefits. I realize good things don't come easy, but if you can't have something you want and it affects you negatively, you need to re-evaluate your reasons for this particular desire. Accept the response given to you and be patient. Life will provide you with something better later.