Ever since he started dating this girl...?

One of my close guy friends has recently started dating this girl. I was happy for him and at first she seemed really nice...but lately he just hasn't been the same. for example, he used to crack non-stop jokes and just act like a total idiot and not care, that is who he is. anyway, since she has come into the picture, he has been really serious and not very chatty. I dont understand, why should he change because he has a gf?