Why I Lose Interest

This is my first official musing. The one I just got in the shower that inspired me to have this blog.

I am a fairly attractive guy. I am fairly outgoing and I usually appear to be confident in myself.
I used to be very insecure and had low self esteem. I feel I have mostly conquered this over the last 3 years, but hints of it still invade my thoughts and actions now.

A few weeks ago I met a girl who is very attractive. This girl, before last night, I would have listed as "way out of my league" to use a technical term. I still liked her a lot, but wasn't holding out much hope for anything to happen.
Then last night, she was at a bar that I frequent quite a bit. She called me to invite me just as I was walking in the door. This is insignificant except for the fact that "she called me" when she was "sober". I walked over, said hi, and sat down for awhile. I am no dummy, I've seen Swingers, and I have played "the game" for quite awhile, so in an effort to not appear too eager, I left the table for awhile to talk to some other people I know(had a good chat with my ex, but that's another story entirely). I even danced with a girl who is always there that I usually dance with.
A quick note on playing "the game": I don't know who wrote the rules, why we all must play by them, or why it usually works, but I hate to think of myself as a true "player". I only follow the rules to get by(though lately I sometimes feel like I am becoming a womanizer).
So I am spreading myself around the bar, not to make her jealous or anything, just so I don't appear too attracted to her, because according to the game, that is a turnoff.
Long story short, the evening proceeds, I play it cool, eventually I am dancing with this girl. The bar closes and we part ways with a semi-awkward kiss goodnigt. On my way home, she calls and tells me I should stop by her friend's apartment "to say hi" before I go home.

Ok.

I do so, and soon realize that I'm staying for longer than "hi". We watch a little TV, talk a bit, make out quite a bit, and then she falls asleep on the couch with me.

I am coming to my point. At some point, while making out with this girl(who I am incredibly attracted to), I notice something. I am ALREADY losing interest. How could this be? I have often pondered on this point, and have come to a few conclusions, in no particular order, save for the last one, which just came to me in the shower an hour ago, and I believe is the most profound.
1. Guys and gals both enjoy the "thrill of the hunt". It's all fun and exciting if you don't know whether he/she likes you or not, assuming you both enjoy the ridiculous cat and mouse of "the game".(Probably true in many cases, but I honestly don't know if the ladies do this at all. Any input is welcome.)
2. I am admittedly terrified by relationships and commitment. This could just be my initial instinct which tells me, "This could work out. RUUN!!!!!!"
(I am a strong believer in this one. It's true and........well......it's true. Period)
3. Here's the new one. Before I found out that she liked me, she seemed unattainable. The moment I was shown otherwise, I thought to myself on a deep sub-conscious insecure level, "I am not that attractive/cool/good enough. If this chick is going for a guy like me, there MUST be something wrong with her. Time to move on." As if liking me is already a strike against you.
It reminds me of the Groucho Marx quote, "I would never join a club that would have me as a member." I am not the first one to use this quote in respect to romance, Woody Allen used it in Annie Hall way before me.
(This, being a new one, I'm not sure of at all.)
Feel free to comment at your leisure. Any and all input is welcome.

And just for the record, I am going to continue to see this girl, I am only wondering why I think the way I do.