I think it often starts from bad advice, or bad instincts. Young men usually do not know how to act around girls. Often, they will get advice from friends or older siblings that the way to keep a girl interested is to make her jealous. This advice seems to stick around, no matter how dumb it is.
While it is not exclusive to one gender, it does boil down to insecurity. If we believe that the other person cares for us, we don't need the validation that making someone jealous might bring. Yes, it's possible that the other person doesn't care enough for us, but trying to make them jealous to prove it is still insecurity. We need to validate ourselves and not seek to be validated by others. That validation is nice, but it should not be necessary.
I would agree with the above posts that this is not an issue that is exclusive to men. In my experience, girls do it just as often, if not more often.
I think it speaks to many things. First, it shows general relational immaturity and/or insecurity. Though plenty of people do it for malicious reasons, in my experience, it stems more from something internal and an attempt to achieve something external.
In a more serious relationship (marriage for example, or a long term relationship that goes beyond the youth of HS relationships) this kind of behavior is often most indicative that the person inflicting the jealousy has unmet needs. For women, it is most often a need for love and/or affirmation. For men, it is most often a need for respect and/or physical affection.
To many young men (and women) jealousy shows that a person cares. Unfortunately this is often true with grown people who should know better. They think if a person is jealous or protective it means they love them or the opposite, if it doesn't bother them then they must not care. Realistically though, jealousy is a destructive emotion, to the person who feels it and the relationship it affects. While some people are jealous by nature and others aren't it isn't a sign of love. A good healthy relationship means trusting the other person. Unfortunately when you are just starting out with the opposite sex, it is hard to be confident in your relationship, so you make someone jealous to see if they really care about you.
Sounds like you are dealing with this personally. Just remember to make sure anyone you are with treats you with the full respect you deserve. =)
Depends on the guy, I'm sure. But one reason might be because they want to prove to themselves that they are attractive. It's kind of perverse, but sometimes when a guy gets one girl they start to worry that maybe no one else could find them attractive. So then they flirt as a way to test that, to see if other girls might like them.
Also... I don't think girls are beyond doing that themselves. At least not from what I've seen watching my students over the years.