I'm the other girl

I currently live in a small town, and about 3 months ago, I was hanging out with a bunch of friends. There were a number of people there that I had never met, including one guy, let's call him Derek. Derek and I really seemed to hit off, and living in a such a small town (1500-2000 people), we had seen each other around, just never had officially met. At this time, I was very aware that he had a girlfriend. We share many mutual friends, and we exchanged numbers that night. Reason for this was because one of his good friends, I have had like this ridiculous crush on forever, so he was going to attempt to set me up with him. Later that week, a few days later, he text me, asking me if I would like to go for a drive with him to run some errands. I was skeptical because I wasn't sure what his intentions were. I have never been the type of person to cheat on someone or hook up with someone who is already in a relationship. I agreed, and went for a drive. Long story short, he kissed me that night, and I let him. Since then, things have heated up, and we have been hooking up. When we do hang out, sometimes we are just chilling out, talking, laughing etc. In a sense, I do feel guilty, but on the flip side, I am having fun. But I know that I shouldn't feel this way, and I should be ashamed of myself. I'm not looking for any sort of lecture from anyone, I realize that this is something I should not be doing. But I can't help but wonder why he's doing what he's doing. I don't think I will ever quite understand. And I'm kidding myself if I ever think this will work out...right?

What's he doing?
1. He's cheating on his girlfriend - he's not very trustworthy is he if he'a capable of doing this.
2. He's trying to get in your pants - and you,re going along with it even though you know that he has a girlfriend - you're not showing solidarity with the sisters are you?

I think you'll be perfect for each other.

I have a feeling that like every fool dealing with a crush you're now hoping for answers like "maybe he likes you and he'll leave his gf for you". We're so blinded when we like someone. >_< The thing is, he might like you, he might leave her for you, but do you want a relationship that started this way? He doesn't respect you or his gf enough to end things with her first, so do you really want him?
As for why he's doing it - who knows? Maybe he's looking for someone to spice things up for him, maybe he's the cheating type, maybe he's actually miserable with her and wants to end things... There are many possibilities, but you shouldn't stick around to find out which one it is.

He's cheating on his girlfriend. Why? Besides being an unconsiderate douche, he's probably bored in his relationship and feels like he needs a challenge. You're that challenge.

But if you know that what you're doing is wrong, just walk out of it. Of course it's easier said than done, but this is clearly a guy who can't be trusted and not relationship material. If he's doing this with his girlfriend, even if, one day, you become his girlfriend, it's likely he will do the same thing to you. Would you like it if some other girl knew he had a girlfriend but still fooled around with him? You are both hurting somebody else, I can't see where all the fun is. Maturity and respect are fun. I know you don't want to be lectured, but I can't understand how you're still letting this happen when you know what you're doing is wrong. That's even worse than being completely oblivious, because at least your actions would make sense.