Jealousy has often be alluded to as the green monster, but is it really a monster, or something we all long for from others. When your significant other gets jealousy, don't you get a little happy inside. Just from knowing that they care enough. But it is an unending battle within you when you're jealous and you cannot show it. You don't want the person you're jealous of, to know they're hurting you. That they are winning. But you want to be able to express your jealousy to someone. Jealousy in this sense could be called the green luxury. Something you admire when somebody else has, but you yourself never appreciate when it's yours. How picky we are as humans.
Let's all look at the stereotypical relationship of the dating world from a girl's perspective. Guy hits on Girl and get's Girl's number. Guy asks Girl on a date. Date goes well, and another date is set. Within a few weeks Guy and Girl are dating. After a set period of months Guy breaks up with Girl for problems that began showing in the relationship awhile back. Girl hates men, Guy gets re-tagged as a scumbag. Girl's friends say "Told you so!" Girl goes through this situation for the rest of her life until she finds the one, and (hopefully) he's different.
Now let's look at the middle part. "Girl goes through this situation for the rest of her life..." Nice Guy then always ask, "Why do girls always go for losers. They act like they can get any girl, then they treat the girl's like crap, and the girl's are hurt. BUT THEY KEEP GOING BACK!" This situation is obviously upsetting to the "nice guys", but what are they doing about it? Complaining? Have you ever heard of complaining getting you anywhere in life? (Don't be smart about that question) If you don't want the girl's to go for the losers, (and trust me they want the nice guys) then put yourself out there, because I guarantee you, if Girl had to choose between Guy and Nice Guy, she'd choose Nice Guy every time. But only if she knew he felt the same way.
Now girl's, keep in mind, a guy may act like the rebellious, un-caring loser, but he may actually be a nice guy putting on an act to impress you because he thinks that's what you like. Make sure you two are completely honest with each other. If he acts like a loser, then make it clear that's not what you want. Tell him what you want, and if he is still a loser, then that's his true personality, but don't jump the gun and assume that he's just a Guy and not a really a Nice Guy, because of how he acts at first.