People want what they don't have. It's very alluring until it becomes attainable. Then once they finally have it, they don't remember why they ever wanted it so much in the first place.
I can't tell you how many friends I've had who have whined to me about how single they are and how badly they need someone only to tell me how lucky I am to be single once they finally get that relationship they so badly wanted.
People think they know what they want, but more often than not they don't. Especially when it comes to relationships.