I do this, but I wouldn't say I laugh at every single thing. It is either because I don't really know what to say back or I think what he is saying is well...bullshit. Sort of like, "Wow, the things you say are so phony I am speechless so I'm going to laugh because I don't know what to say."

So, I don't know what you are talking about with this girl, so it might help us to know what exactly you are saying to her that she finds so amusing.