Why do people want what they don't have now?
There are many examples of people wanting what they don't have. For example, a single man may want to have a girlfriend, but after some time of having a girlfriend, he may want to be single again. Or for example, people in the city may want to live in the village and villagers may want to live in the city.
What causes individuals to desire something they don't have and once they have it we desire to have something else -- or something which we left behind a while ago -- again?
Your examples are all about choices between alternatives that have different advantages and disadvantages. If you live in the city, there are a lot of things to do, better infrastructure, but no quiet and clean air. In the village, things are reversed. Obviously people want both: quiet, when they feel like quiet, action, when they want action, clean air all the time, and good infrastructure. The underlying cause is that people want *both* alternatives at the same time. Your question needs examples where alternatives are not the reverse of each other and not balanced regarding (dis)advantages.
Thanks for input... actually, it was meant to discuss exactly these alternatives. Why you incline to "go back" once you left something because you wanted to have something else.
The reason for desire or want or liking is Pleasure. Various people have various things to get pleasure: from Money, from Power, from Love etc. etc.
But this pleasure is always based on comparison. For ex: I may have money, power, wealth and love. I am satisfied with all these. When I encounter another person who has all these things but with another proportion, then I tend to "HAVE/POSSESS" that. The reason could be, "THAT" appears more pleasurable to me than what I have.
There is no absolute answer to this question 'Why man seeks pleasure? or Where it comes from?'. We may need to understand the nature of pleasure, how it occurs etc etc. Read this link and you may understand scientifically what is want/desire/liking/pleasure.
There is also a philosophical answer to your question which is related to mind. As put by Hindu philosophy, Mind always seeks pleasure. This pleasure, found in external things, is always in the form of self satisfaction. It feels happy with possession of things outside.
But once it turns inward and looks at itself, it understands its nature. This comes out of intelligence. Then it gradually seeks inner calmness rather external cravings. Resting in inner peace and calm is more happier and blissful to Mind. But it is under illusion that it finds pleasure in things in the external world. The reason being mind, as put by Hindu/Buddhist philosophers, identifies itself with the body and it thinks it has a separate individuality - "I", "ME","MINE". When there is a separation like this, automatically there is a comparison. When comparison exists, desire sprouts. You better read about mind in Google. Lot of notes and articles are there.