I'm exactly like that, I get very bored real quick.
It's like once the challenge and exitement's gone, I quickly move on and it's not something I very much like about myself.
I've hurt a few people with this process and have no idea why I do it. I do try and be more careful in those situations, but like garg said it's kind of a built-in trait.
I guess your mind just always needs something to work towards, to keep you going.