Why would a guy be quiet with one girl but talk a lot with another?

I used to go out with this guy and i swear I had to force all the conversation, and that was hard considering i'm naturally a quiet person. I just talked to the girl he 'dates' now and supposedly he talks too much when they hang out, and she's a pretty loud person so thats saying something. I was kind of surprised. why would he have been so quiet around me rather than her?