Why do people play head games?

Head games being any action that brings pain mentally or physical to someone else, just to be cruel or have control. In romantic situations of course, it seems to me that every girl I date happens to be obsessed with drama. I know this is just a poor taste in character on my behalf. But still as to what means to people of any gender, feel this silly need to mess with someones emotions and play games with the heart. It's one thing to be honest, like to be honest I would much rather prefer someone to be honest with me before they went and had relations with another person while engaged in a relationship. For example, I have a ex that always talks to me about her and her new boyfriend. She isn't really a happy person to begin with so I try to be nice. She likes to talk about our past and such and tells me she wants me back and so on so forth. My outright concern to this is, she had been asking me to do some things that would be offensive to the partner if they where ever to find out about such things. It makes me wonder, when I was with her. Was she talking to her ex's and requesting the same treatment from them without me knowing and pretending like it was ok to be romantically involved with someone but to crave and lust for another person of a past relationship; but never act on these statements that she proclaimed to be true? Is it that she is just simply not satisfied with life, or does she feel the need to always have someone besides the person she is with, to be interested in her romantically? As for many other people, why must people intrude in another life when they have dispatched of a relationship, attempting any means to make a persons life as miserable as they can possibly succeed at. Making there way into your family and friends trying to turn them against you, just to get back at you for treating them right.... It's very strange how people continue to do these things and how I am yet stupid enough to engage in a relationship without at least getting to see someones true colors first. Lately I have been doing of taking the time to get to know someone and it seems that now that I am older I wish to continue through life having as many civil relationships, or relationship as possible. I am not out to date and enjoy as many people as possible, I wish to just enjoy there company and most of all be able to laugh and have some fun, as well as trust that person. Without the worry of consistent games on a persons head. I have also done a good job of staying away from all the drama, but simple curiosity has always led me to wonder as to what brings on these means of behavior? I am also wondering if pursuing a romantic relationship at the age scale of 20 to 30 is a wise idea for I am not sure if I will be in this town for much longer due to the simple fact that I must pursue a job that pays well and in order to do such a thing I must also leave to attend to schooling. It is possible to get the schooling and acquire a job in this town I am currently at but do I really want to risk doing all of these things and giving up good opportunities for one person that may possible be here as well as my friends? Any opinions would be greatly accepted, but still my question about the head games remains as well. Is it wise to date people who play head games or is it wiser to just hold on tell the right person comes along? Is it better to jump or to take things slow and see if they are worth waiting for?