Do hormones alone make you fancy people?

Right, I'll give an example as to what I mean. My friend and his girlfriend have been together a long time, and they really like eachother. But recently, he's started having feelings for another girl. He claims he still really likes his girlfriend, and that although he really likes this new girl too, he thinks it's just down to his age (17) and therefore his hormones, and that it's just a ping thing, and that he'll go off her soon. I know that hormones can send male drives into overdrive, but I'm not sure about liking personality and stuff too. So, is it just his hormones, and a ping thing, or is it most likely something more?