Dealing with jealousy in a relationship can be an incredibly challenging endeavor because jealousy is among the ugliest of emotions, and can cause the worst of behaviors in people. Your best bet is always to recognize jealousy early in a new relationship, and then immediately address that behavior with the person. And if the jealous person cannot control the issues that usually reside only inside their own head, then it is often better to simply leave them behind, and find a different girlfriend or boyfriend.
Signs of Jealousy
I surveyed my own past (which includes stints of dating several very jealous women), along with my friends, and what I came up with is this short list of common behaviors that indicate jealousy. So you should watch out for any one or more of the following:
1) Calling you multiple times per day to say "I love you", but really she (or he) is just checking up on you.
2) If you don’t answer one of those calls, asking you what you were doing and why you couldn't take the call.
3) Going through your cell phone, and seeing who called or texted you.
4) Going through your email, and reading it. Maybe going through your car, or any of your stuff - including your calendar or day planner.
5) Insisting that you do not have friends of the opposite sex.
6) Accusing you of looking at someone of the opposite sex.
7) Getting mad that you were talking to someone of the opposite sex, and giving you the third degree of questioning about him or her.
8) Showing up unexpectedly at your place of employment or just following you around without your consent or even your knowledge.
9) Jumping to conclusions and perhaps getting ridiculously angry about anything having to do with any of these signs of jealousy.
10) Reacting with violence as a consequence of conclusion-jumping. This could include physical abuse, or breaking things or throwing things at you.
Remember that if you are in a brand-new relationship, everyone is often on their best behavior. Those signs of jealousy might appear gradually and could even fly under your radar for a while, so your best bet is to keep your eyes open for them.
Why Are People Jealous?
It really doesn't matter WHY someone is jealous because you (the man or woman who is dating the jealous person) still has to deal with it. Most jealous behavior is a consequence of insecurity. And insecurity can result from having had someone cheat on the person in the past OR it could be because the jealous person themselves has a history of cheating. Sometimes the jealous person can also be a control freak which means they feel like they have to control everyone and everything around them. I'm sure there are other possible reasons for people becoming jealous, but I really think that insecurity and/or being a control freak are the top two.
What to Do When You Are Dating Someone who is Jealous
I'm not a psychologist, so the advice here is simply what I would do.
1) Identify any jealous behaviors in your partner as early in the relationship as possible.
2) Discuss that jealous behavior with him or her, and help them to recognize it. If they can admit that their behavior is jealous and unfounded, then you have made some progress.
3) Reassure him or her that you are not going to cheat.
4) Explain that all relationships involve risk, and that being jealous and/or controlling does not reduce risk rather it increases the likelihood that the relationship will fail.
5) Be firm and explain that the jealous behavior you have seen in them will definitely drive you away.
6) If the jealous behavior persists or increases, you should end the relationship because problems like this that cannot be addressed early on are more likely fairly deeply-ingrained and will likely grow worse.
Now not all of what I just mentioned should happen during the course of just one discussion, but (for me) it could definitely all take place within one week. You might have a different pace. But I would not suggest dragging it out over more than one month max because life is just too short, and if you need to move on it is usually best to do so sooner rather than later.