I think some twisted reason- people get a kick out of it. I HOwever they fail to realise that if they manipulate others they are gonna be doing it to themselves first.
Yes, power and control over someone else as Stella5555 says. Usually they are searching for some kind of acceptance and are not in control of their own lives and selves enough to live freely and let others live freely. They are not the best people to fall in love with, be friends with or even be around. Having said that, no person it one dimensional and so I think that people use manipulation for various personal reasons, which can be worked on and dealt with. I think some maniulative people are wonderful people and become trapped by their desire to control and become freaks...
Some people like to manipulate people for the sheer feeling of power it gives them. Obviously this is done by people who are really screwed up emotionally.
Happiness, they hope.
What constititue people's happiness though? Long answer for some, shorter one for others. Control freaks wouldn't stop until they can control you like their TV. Money freaks wouldn't stop until their last breath. Sex freaks wouldn't stop until they can't do it no matter what pills they take.
I would define a beautiful relationship as one where two ALREADY happy people, who are ALREADY highly compatible, meet. I have seen a couple of these to know if you work on it, you would get one.
some people are just willing to do what they have to do , to get what they want . so there manipulating them to get what they want