I used to think that those places in the OT where God called Himself "jealous" was an unfortunate anthropomorphism that put God in a bad light and made Him an easy whipping boy for my atheist friends. In some ways it was a bigger problem than the problem of evil, because we have God actually admitting to a negative human trait, not so much dealing with a syllogism where we can (rightly in my view) insert "mystery" as the easy answer. But a recent conversation with the chaplain at Oberlin College changed my mind on this.
We were having a chat one day in his office a few months ago and started telling me that jealously is not bad, and that in fact, it was even a necessary and appropriate emotion, I immediately balked. I was certain that the scriptures spoke against jealousy. He stuck to his guns saying that the scriptures speak against envy, but there is no biblical injunction against jealously. He explained this by saying that envy is all about coveting something that does not belong to you. Jealously is about being indignant about someone whom you are in covenant relationship with consorting with another.
It is wholly appropriate for a woman to be jealous when her husband is cheating on her. He made a promise to be with her only, and not with anyone else. Further, she is not only jealous for her sake, but for the sake of their relationship and for their children. You cannot truly love someone and cheat on them at the same time.
When i started thinking about this argument, it began to make sense to me. I then began to apply this to God and His relationship with us. I now understand that is also appropriate for God to be jealous of us when we forsake Him because He alone is our creator and we have a covenant relationship with Him and with no other. If He were not jealous then He would not care. Just like a man who does not care about his marriage enough when his wife consorts with another man and does nothing about it, and perhaps uses it as an excuse to commit adultery himself. No, if God could not be jealous, He could not care. It is not enough to say that He shoud be happy because someone else can make us happier. That argument may or may not work in human relationships, but it cannot work with God because ultimately, the best gift that God can possibly give anyone is Himself. Nothing else comes close.