This is an interesting topic, and something I haven't given a whole lot of thought about until recently. I've been analzying my feelings and what takes place within where I go from one extreme to the next, and I still haven't figured it out completely (I think I'm making some progress).. Because it's more than one emotion taking place and I'm having a hard time untangling them. The best I can gather is something like this...

For me.. It almost always involves a girl, usually attractive (not always the case, because it's occurred online with girls I've never seen)...
The common denominator for me... Is that connection you have with someone, and the feeling of being liked, appreciated, and they enjoy my company...

This creates a high for me, because I'm LIKED, and their Great and I perceive them as being better than me...

From here other emotions that eventually occur are Jealousy and Fear. I see that person as being more likeable than me, better than me, and easier to make friends than me. (I suspect this is where the jealousy and fear come into play). I become threatened that because they possess all these qualities that I don't have (jealousy), they'll meet new people, and move on, and I'll be left alone yet again (fear).

Then combine that with my black and white thinking, and it becomes more difficult.

I'm beginning to realize in my head though that the person isn't better than me. I'm more aware of what's taking place within me, and trying to keep the relationship healthy. I've yet to overcome that feeling of them being more likeable or having it easier making friends. I "think" I'm becoming less impulsive with my actions, and maybe even a bit more rational then emotional.

This is the farthest I've gotten in figuring out this process that takes place within me, and it's developed a bit more than a few days ago.. Still trying to figure out the rest though.